Hi. My character is a Devastator, and I had been planning on diversifying him so he's competent in melee, too, and the easiest way to do that was to take Deathwatch Champion as an Advanced Specialty. I've recently changed my mind about that, as I have realised that I always, always try to spread my characters too thin, and my truly effective characters, in any system, have only come about when I've been forced to specialise.

However, I'm still considering Deathwatch Champion nonetheless, for the defensive Talents such as Wall of Steel and Step Aside, the earlier access to Dodge +20, and the two Hero Signature Wargears (Master-crafted Lascannon! Artificer Plate opening up the normal Hero SigWar for something else!). Also I think Xenos Bane, the special ability, works at range? Not sure. Maybe later, much later, I might take some of the melee-based stuff, get it after I've got all the ranged stuff I'm interested in - if my character lasts that long, of course!

Does this make any sort of sense? The only other Advanced Spec I'm considering is First Company Veteran, but all the skills are Intelligence-based, and Int is my worst stat - plus, could someone explain to me how to make Battle-Hardened and Survivor useful? I know they are, I've seen people rave about how good they are, but I seem to be missing what everyone else sees.

OK, a little late (my fault for not noticing this post earlier, sorry!), but I'll try to answer as much as I can. Bear in mind that 2 DW campaigns can be vastly different, but I do have a (although melee-oriented) Champion in one of my Kill-Teams, so I hope it'll help.

Deathwatch champion is at the same time best vanilla Advanced Specialty (only Honour Guard and Storm Warden's special one are better, they are basically Champion+), one of the most expensive with both high entry cost and high cost for abilities, and a Specialty that's mostly of no use to Assaults (since they have basically all of it on their Rank, and cheaper, and can take Champion only for flavor and some special abilities).
On the other hand, Devastator Champion is the best possible vanilla combination aside from some Librarian builds. If you already bought all stuff from your Devastator Rank, up to whatever Rank you are currently, and some supporting Talents and Skills from General/Chapter, AND you can see that there is nothing revolutionary in the next 4000xp range for you, take Champion. Access to Wall of Steel, Step Aside and all other Melee-Talents combined with having one of the best Melee Talents, Stalwart Defense (that Assaults for some reason do not have), a TON of cheap wounds on Devastator Rank to support Xenos Bane and you are shootier than anyone and as good (and sometimes better) in Melee than your Assault brethren, although with a tad lower WS and it costs you an arm and a leg. Figuratively, of course, right?
On a third hand, which I just grew in a totally not-heretical way, it also depends on your utility and Chapter. If your are a skill monkey in your group, and you are full of Lores/Demolition/Tech-Use and such, than you probably have a lot of stuff to buy, and Champion would actually decrease your usefulness in a team. The same applies if your Chapter has some amazing abilities that work well and create synergy with some options from General or Deathwatch Table, as well.

Long story short' if you can wait and there is nothing urgent for next 4000xp, do it. If you need to buy stuff constantly to fulfill your role in a Kill-Team, either don't do it, or split your XP 50/50 or something like this, with one part spent and other waiting for better moment to buy Champion.

I would talk with my GM about changing Xenos Bane to work from Melee-only to Ranged-only, if he agrees. It sounds reasonable for you, although he needs to consider balance.

I also agree that Devastators have very weak access to better Specialties. If you do not want to be Captain, you're left with Kill-Marine (limited use in a Kill-Team, although it can be valuable but rulebook limits it to only Rank 1, although I allow it to be taken alter), First Company Veteran (a very general rank with not much to it), Intelligence- and Fellowship-based Keeper and that's it aside from chapter-specific ones.
However, you can point your GM to a special, surprise Specialty available in Outer Reach. I do not want to spoil the fun, since it's amazing class, but if you're a player, you should direct your GM to this rulebook. There are possibilities for Ranged characters there, too! My shooty Tactical with Bolter Mastery and Astartes Weapon Specialization (Bolter) was very happy when I gave it to him (although you will need at least a little bit of Intelligence to use it to fullest extent, you can buy it now and than slowly rise Intelligence), & there is also a new special bolter ammo there, bunch of interesting Relics and an epic, funny editing mistake (with one Relic granting 10, yes, TEN Renown per Objective completed XD). For GM, there are also some new enemies, few good missions and a ton of plot hooks. My players spent time from reaching 20, 000 to achieving 50, 000 XP on basically this rulebook alone, if you're flexible you can make a whole campaign out of it.

Survivor, it's the ability to once per mission nullify all penalties to Characteristic Tests, right? I do not have rulebook with me, so going from memory, I'll just write down use of it for every Characteristic (although, from what I remember, some Characteristics cannot be chosen for Survivor). Also, as a GM i think of Survivor as something more than usual, and allow some special uses, normally impossible, like lifting something :

WS: Penalties are very rare, but Overwatch, aiming for specific body parts and Parrying while laying in 1m deep snow in a middle of exploding battlefield in the middle of a night with no light, or hitting someone in a situation like this, come to mind.
BS: The same as above, although bonuses for BS are easier to gain, but Hard Target, Sprint, and Flyers also exist.
Strength: Lifting something REALLY heavy, for example closing doors to a bunker, or lifting a vehicle, or throwing someone heavier than you can (but remember, no one throws a Squat!, I mean, a dwarf), Grappling large targets. That's just from top of my head.
Toughness: Resisting Poisons, although Astartes are really god at it (but remember, high-level Dark Eldar Posion used by Master-Level Enemy, high-power Nurgle stuff and some Psychic Powers can give you modifiers like -60), and the most important - resisting Radiation. Difference between passing first Toughness Test and not passing it, losing 2d10 Toughness with no Reduction and THAN rolling again, and again, until you leave the irradiated area, can be a difference between life or death, and radiation penalties can range to even -60, and theoretically it can even deal 4d10 Damage, if you're sitting in a reactor without a Terminator Armor, of if enemies make a very good traps (happened to my players once in a starship and once in underground base, nowhere to dodge in tight corridors!).
Agility: Penalties from Disengaging via Acrobatics Test; penalty from Tyrannic War Veteran's special magical dodge; penalties to Drive, to Evade while driving, penalties to Pilot, including flying with Jump Pack through a gigantic storm made of pure daemonic hatred and such.
Perception: Penalties from camo cloaks and such, from darkness, from psychic powers, from special Talents that give penalties to Scrutiny.
Intelligence: Finding obscure information is the most obvious; some extremely hard Tactics tests, especially Opposed. There are also some Intelligence-targeting Psychic powers, and they are really nasty.
Willpower: Psychic Powers of all sorts. Possesion. Daemonic Presence.
Fellowship: Fear and Cohesion damage tests. Interactions with stubborn or hostile targets. There are some other possibilities.
Chapter about Skills in rulebook gives a wide variety of Penalties in every Skill, there are more examples there. It can be a kind of rarely used, but life-saving ability. And if I were you, I would suggest to your GM that you can use it AFTER testing, to make it more viable and not waste your once-per-session bonus.

Sorry for being late, hope it was useful!