Has anyone tried using the Aptitude system for Characteristic/Skill/Talent advances for Black Crusade characters? If so, I'm interested to hear your thoughts on adding this bit of extra complexity.

I'm guessing that the Aptitude system for Only War characters is meant to be analogous to the Alignment system for Black Crusade characters. However, as page 312 in Only War suggests, Only War characters could gain enough Corruption and essentially become Disciples of Chaos in Black Crusade.

Therefore, would it be better to retain the Aptitude system in parallel to the Alignment system now in effect, or simply replace Aptitude with Alignment?

Porting the aptitudes would certainly be possible, but I'd refrain from doing so. Alignment and its benefits and drawbacks are among the central mechanics of Black Crusade. Consider a guardsman (a simple weapon specialist) who has discovered some forbidden texts and did not turn them over to a priest or commissar, keeping and studying them instead. In Black Crusade, that guy is on the fast track to becoming Tzeentch aligned, possibly even getting Warpsmith or the Mark and learning sorcery. With Only War aptitudes, he's in for a lot of pain as he pays through the nose for every single advancement.

Alternatively, one could think about letting the characters keep their aptitudes right up to the point where they become aligned to a deity, whereupon they switch to the alignment mechanic.

There is a possible way to use both if you want, use aptitudes untill you are alighned and then sum them both up

lets treat 2 aptitudes as true, and 1 as unalighned, then:

opposed+true=unalighned,

unlighned+true=???

unalighned+opposed=??? (profit)

so you have to decide what is more important here.
That would take some work and could possibly create 2 more steps of xp costs: true-???-unalighned-???-opposed.

I built a system for this and it seems to have been quite successful.

First, our group has been playing 40k RPG of various forms since Dark Heresy first game out, and we play all 5 game lines (everyone has the stuff for Only War, we just haven't started an Only War game yet, but several are in the works). When Black Crusade first came out, I started building characters in it to see how they stacked up with other game lines and was rather disappointed. Each time the mechanics have been updated (DH to RT, RT to DW, DW to BC), we've imported the stuff from the new system that we liked (which was generally most of the mechanics). Unforfunately, the xp costs for everything seem to have crept up to ridiculously high levels (highlighted by the fact that we have characters from DH that are at high ascension level, rank 14-17 now).

In DH, things that your class was supposed to be good at were generally 100xp. In RT, classes are far more of a straightjacket than they were since there are no branches to the paths, plus talents that you should have by Rank 8 as say, a guardsman, are held back until Rank 12 as an Arch-Militant. It got even worse in DW with things that you're supposed to have being even more costly than RT, and marines having to wait until Rank 16 for certian talents that Guardsmen got by rank 8-10 (I'm going by universal rank here, not the renamed ranks with the same totals from DW).

Since everyone wanted characters balanced against their existing Dark Heresy characters (of which we have a spread ranging from rank 3 to rank 17 with about 6 different acolyte cells), I started making a priority system for adjusting costs that was eerily similar to what was released in Only War. Once Only War came out, I mercilessly looted it for ideas for my freeform system and never looked back.

The basis is thus. You choose affinities not unlike Only War (although since things have been so freeform, the selection for backgrounds is starting to resemble GURPS). We've experimented a bit, but have settled on 8 affinities for a character. Each skill, stat, and talent has 3 affinities listed next to it. For each matching affinity, it lowers the cost by one step. Additionally, a significant number of them have a chaos affinity attached to them that can also reduce the cost by a step, sometimes two. Every advance has a cost number next to it on a big spreadsheet.

To simulate how some things increase cost each time you buy them, stat buys go up on the chart by two steps each time you buy one, but are open ended so you can buy as many stat advances as you like. Openendedness is a deliberate feature here. Skills go up in price each time you buy them, and some talents like Psy Rating and Sound Constitution go up each time you buy them as well.

The main cost tree is a 10-step pattern of 100, 125, 150, 200, 250, 300, 400, 500, 650, 800, then starting again at 10x the cost of the previous (1000, 1250, 1500, 2000, etc). This way things eventually become prohibitively expensive but are never barred from purchase. I also have a bunch of new talents that we've hatched as various character concepts have suggested - improving suppressive fire, ability to assess a battlefield more quickly, variations of the ascension talents that make more sense, ability to dip into Deathwatch squad abilities, etc.

If anyone is interested, I have a spreadsheet for it.

Edit: One other important change - acts of devotion. Buying advances aligned to a particular god does nothing. Committing acts that please a particular god are what aligns you to a particular god. As it currently stands, there is no incentive for a Slaaneshi character to indulge in acts of depravity, as that will only gain corruption, thus shortening the character's life for no benefit. If such acts count toward becoming aligned and getting a free mark, then there's a reason to want to carry out these things. Once you become aligned, you get rebated the xp spent on aligned advances (I no longer have opposed advances either, that proved unnecessary when your actions became what mattered). This actually seems to have strongly encouraged unaligned characters to pay homage to the different powers carefully (much more difficult than just selecting advances that don't tip you) and for aligned characters to continually make sacrifices to their chosen power (since you lose an act after every chapter of the story, forcing you to keep up your devotions continually).

That sounds pretty neat. Wanna post a link?

These are in .txt and .ods format. OpenOffice (or whatever they call the open-source clone of MS Office these days) is needed to use the spreadsheets, unless MS has decided to start supporting that format.

They used to be locked, but too many of my players couldn't figure them out.

http://rapidshare.com/files/4160512267/BC%20Freeform%20WIP130418a.zip

Take what you like, discard what you don't, I don't ask for any credit. I'm making these rules changes because I like the impact that it has on the game. If you don't, or if you find that it results in effects that you don't like, by all means change them. There's still lots of imbalances and tweaks that need to be made. Heck, I'm making changes to them all the time. We've run two sessions so far as playtest with another one upcoming this weekend (which will be the big test - earlier stuff was easily facerolled, but this is the climax of this plot arc and might result in PC deaths).

The basic idea for using these is to read through the house rules document (which assumes that you're already well versed in all of the other 40k game lines as well), then grab a template from the templates document and make your picks. Once you have your picks, (and the impact of any pre-spent XP for games starting at higher level than just generation), then you open a fresh copy of the freeform spreadsheet. The cost schedule is the second page, but it will behoove you to copy those numbers onto paper or a notepad since you'll refer to them a lot. The main page has the names of the advances in Column A, the cost factor in Column B, Prereqs in Column C, and the effects in Column D (which includes a lot of houseruling mixed into them). On the far right, there's a section of orange cells that are pull down menus to select your affinities, which automatically adjusts the costs just to the left of them. The blue cells are freeform to allow you to write in the advance and the cost that you paid for it. Remember that cost increases 1 step each time you buy a skill, or two steps each time you buy an attribute.

EDIT: One important thing - these characters will NOT be balanced with default Black Crusade, but they will be balanced with Dark Heresy and Ascension level characters. Every game line since Dark Heresy has increased the costs for everything - RT gave out universal weapon talents and the like, but doubled the cost of almost every skill and talent purchase, Deathwatch doubled the stat costs too, and Black Crusade blurred the lines a bit, but every advance has been much more expensive in every game line since the original Dark Heresy. Since our group has been playing Dark Heresy since its release, stuff that has been ported in from other game lines has proven to be overpriced, leading to either underpowered characters or needing to adjust XP costs (We had an Arch-Militant who was rebuilt as an ascended Guardsman and ended up far more powerful and versatile with much the same advances). As such, DH has been our measuring stick and we've been forcing everything else to match that.