I'm probably the only one who is intrigued by the mini-review here (and I am not the usual FFG cheer crowd). Of course it depends A LOT on execution but it sounds so much better than Ascension for Deathwatch. Let me quickly run through why:
Chapter 1: Much, much background goodness about what happens to marines after their deathwatch secondments, and how/why some are selected to come back - or not to leave at all. The criteria for picking Watch-captains, are pondered on.
Why isn't this awesome? This sounds like: finally role-playing material, role-playing as in role-playing. Yeah, no crunchy I-can-kill-you-now-even-better which the game doesn't need at all. If any game doesn't need it, then this one. The sociology of the Deathwatch, its rituals and traditions, its mindset, its outlooks... this is interesting material for study and for setting a realistic and rich stage. I have a player who lost all his Fate Points and who is set to retire his PC. This sounds brilliant.
Chapter 2: In the same way Jericho Reach and GM's guide named most of the current command hierarchy and the Chamber of the Vigil, this section lists off seven famous former kill-teams. More than just background - it tells you who the kill-team consisted of, describes each member, tells you what they were famous for, including several pages of their most famous mission (usually a Famous Last Stand [tm]) - and then gives for each one a set of adventure seeds relating to their legacy and (for all but one kill-team) some of their relics that are left in the deathwatch's possession, which your players are likely to give more of a damn about if they know the whole story rather than just 'it's a +2 relic blade'...
Again, I see this as role-playing material. To evoke a sense of history and lineage. As a GM I can weave that into a mission... just when the players are faced with over-whelming odds, the team leader remembers the legend of X, then I can entertain the players with some brief story-telling, give them all a bonus if the leader passes his command test as he invokes the saints name and it makes everything more epic.
Chapter 3: The meat of the 'rules' section. There's no ascension as a marine. Rather you become awesome as a kill-team as a whole. You agree as a kill-team to buy a legacy for what we shall refer to as a "holy bejeezus metric shed-tonne" of XP each. You pick one of the several options. Each member is awarded by his peers in the kill-team a title from a list such as 'breaker of hordes', 'guardian from afar', 'preceptor of shadows', and so on. Each legacy includes five such titles in various combinations, though you can expand them to add more (at a cost). Each title has a benefit to the bearer befitting the name.
In addition, each legacy gives the team leader three manouvres to call upon - one offensive, one defensive and one tactical, which can again be added to for XP to up to two of each. These are broadly akin to squad actions, except for three differences:
1) They don't cost cohesion to use, instead requiring a check by the team leader (usually command but often tactics)
2) They require a very high squad cohesion score to allow the attempt - as in I think about the lowest needs cohesion 10.
3) The results are very slightly awesome. An out-of-turn suppressive fire from everyone in the kill-team at a defined area plus a 'free reaction' overwatch attack from everyone in the kill-team at anyone moving in the same area, for example.
There are also some unique relics for one of the legacies. Including a custom-made stalker-pattern which is essentially a marine-calibre barratt.
This follows a theme I originally came up with on the forums here when Rites of Battle came out: the main theme of Deathwatch being about growing together as a team of marines originating from different backgrounds. In theory this is brilliant but, again, I don't know the execution.
Chapter 4: A (quite sizeable) adventure picking up from the last stand of a famous former kill-team (one of the ones from Chapter 2)
Can't comment on that, of course.
tl;dr = if you expect Ascension:Deathwatch, it sounds like you will be disappointed. But from a role-playing perspective it sounds like it could be fun.