Has anyone here actually used this alternate rank? If so, how did it go? And technically while you aren't total Reaver until you buy the Cold Soul talent, does that talent ultimately change much on the roleplay front?

Reavers are hard to make work in any non-pirate RT campaign. That said, the Arch-Militant in a previous RT campaign that I was GMing was a "reformed" Reaver. So, sort of working his way back to the Emperor's light. That said, he was still a terrible terrible person when it came to combat. His Bio included discussion about collecting body parts and skinning things as a Reaver - so I imposed a WP (-X) test on him at the conclusion of combat to resist taking "trophies." That test got easier as time went on.

One does not necessarily need the Cold Soul Talent to be a total Reaver. Remember, they get another unique Talent: Hatred (All), which makes them pretty much hate everybody and everything (including fluffy bunnies).

To answer your question, yes I have used this alternate rank. My Void-Master is a Rank 2 Reaver (had I known about the alternate rank, I would've taken it at Rank 1, as it fit his back-story to a T).

As far as RPing a Reaver, though I did not give him Cold Soul (...yet), he does have Hatred (All), which creates similar RP qualities if you want to make your character behavior and your character stats match up properly. Simply put, he is a raging *******. He often does things that, per the standards of games like D&D and Pathfinder, would be considered completely evil (such as casually murdering defeated opponents after they're no longer a threat, chopping people's fingers off when they don't answer his questions, threatening civilians with Razorhail guns, etc...). He is immensely unfriendly towards practically everyone, and he spends most of his off-camera (and even much of his on-camera) time getting hammered.

If I could go back and make a friendlier character, I would, because it does get a bit constricting, but I make it a point to RP his talents and background properly. However, he isn't always 100% Reaver 100% of the time. To make him more appropriate to play, he is a recovering ex-Reaver (because it doesn't make much sense to have a full-Reaver in a RT party). Per his back-story, he once was an honorable man before falling into the depths of darkness from certain events, and now, decades later, he is trying to claw himself back out of that pit to be the man he once was. However, he slips back to his darker behaviour frequently, and still can't shake his nasty disposition even when he tries to be friendly.

As far as mechanics go, he has become a murderous melee machine. For quite awhile, my character had to be the prime combat character, as we had no Arch-Militant, and the Ork Freebooter that joined the game at its inception could never show up often enough to be reliable. Frenzy and Hatred (All), combined with a decent WS, can make for a fierce and hardy opponent. That he is a Void-Master, and eventually got a jump pack has made him all the scarier.