Apparently these are pretty popular right now and I've had a chance to look through the first three:
Dark Heresy: You're a fairly low-powered group of characters working for an Inquisitor rooting out heresy inside the Imperium. This does very little for me because while Inquisitors and their personal retinue are interesting, the guys that work for them at this level don't do as much for me. Plus, the things that you're investigating can get very powerful in a hurry and I suspect this would play out a lot like a Call of Cthulu game. I'm sure some peoepl would be interested in that but not me, and I think the popularity of this game was largely due to it being the first 40K RPG ever released rather than the awesomeness of the premise. Beyond this, I just don't think the spirit of 40K is in sneaking around uncovering mysteries - that's too much 90's conspiracy x-files game. The 40K part of the story begins after the planetary governor has turned to chaos and declared a rebellion and the sky fills with marine drop pods streaking towards the planet.
Rogue Trader: You're the command crew of an an interstellar trading ship sent to the borders of the Imperium with a warrant to trade and interact with Xenos cultures. Now this one is interesting - tons of freedom to do as you wish, enough power to not be overwhelmed the first time you find something interesting, and all of the cool background of the 40K universe to play in. It does have a bit of the Star Trek problem in that one player is supposed to be the ROgue Trader and theoretically in charge, but I don't think this would be a major issue most of the time. I like this one a lot.
Deathwatch: You're a Space Marine, from one of just about any chapter, and you've been assigned to an all-star team of space marines that deals with alien threats, primarily through violence. This is the highest powered game - marines have 20 genetically modified implants and organs, extensive training, access to the best weapons, and then they get power armor - and is centered around commando raids and assaults on things. It's a very military flavored game, but it also looks to me like it would be a lot like a super team - some rely on their "natural" talents, some are gadgeteers, and some have psychic powers - where dramatically overpowered beings invstigate and solve mysteries using a lot of violence. The small size of the team, differences between the chapters, personal rivalries, and competition make this a little different than a Traveller merc unit or even a Twilight 2000 player character group.
There's a 4th game out now called Black Crusade that deals with playing chaos marines, traitor marines from a civil war a long time ago. I haven't seen it yet but I assume that (much like the Chaos Marine Army in 40K itself) it caters to those players who want all the badaceness of the Marines without having to play the "good guys" - Heh.
The mechanics are the same as the Warahmmer Fantasy RPG second edition from a few years ago so it's pretty much all a percentile-based roll-under system. It's not as flashy as some, but it works, and the DM is encouraged to use a simple list of modifiers to run the game, from a -60 (Hellish) to a +60 (Trivial). Character stats are rated on a percentile scale and then skills allow a character to use the related stat to roll for a task. There are also Talents which are similar to Feats in d20 games, allowing a character to do something outside the normal rules. The whole thing is fairly clean and for the most part does not get bogged down in a bunch of subsystems or special exceptions.
I have to say I was pretty happy to see that these were finally coming out when they were announced. A 40K RPG has been a dream of many of us who have played 40K for a long time, and many, many homebrew efforts have been made, from custom systems to GURPS to Savage Worlds. Basing it off of the WFRP mechanics keeps a certain level of flavor (since the miniatures game 40K was based on and is still similar to Warhammer Fantasy Battles) and makes it fairly easy to translate the vast list of creatures and gear from 40K over to the RPG. The vast amount of lore in the 40K fluff gives the DM a lot of material to use for a campaign too. This is especially true if one has had some of the various side games published over the years: Battlefleet Gothic yields a lot of information on how ships and systems and fleets work in the 40K universe. Necromunda covers a lot of background on Hive Worlds and Gangers. Even going back to the original Rogue Trader game book that stated the whole thing gives a lot of information on creatures and terrain and general weirdness that might be encountered. The interlocking web of support of both useful game material and background details is probably only rivaled by D&D campaign worlds and maybe published universes like Star Wars and Star Trek. There's a ton of it, and when you get out to the more fringey material like the old Realm of Chaos books, there's a pretty good chance more recent players won't know anything about it.
So am I starting up a game of one of these? I'd like to, I'm just not sure where to fit it in. It will likely become one of the rotating "when we feel like it" games with the Apprentices. We have made characters for Deathwatch, and we will do the same for Rogue Trader, and we will play at least an introductory mission to get a better feel for it. After that, well, we will see.