I purcahsed Ark of Lost Souls for use in a Black Crusade campaign, rather than Deathwatch, and expect to have to adapt heavily when it comes to the detail of stats etc, but then I tend to run combats and campaigns in quite a narrative style so this isn't much of an issue.
As a stimulating environment for adventure the content hits home quite well, with opportunities presented for much diplomacy and influencing as well as for ass-kicking. The content straddles both a single adventure-like exploration and the much wider material for generating 'random' future escapades aboard this (or with adaptation, any) space hulk.
There are some nice options, with a new and terrifying alien race (well, new to me at least...) and some familiar but no less scary opponents. I particularly liked the way that the text makes the Tyrannid threat sound really frightening.
The main adventure encourages you to make a selection between exploring different major factions / enemies on board, and these are all suitable for adaptation to any party. Instead of trying to stop an Orc Waaagh! I can see my Black Crusade chaos marines trying hard to ally with or control it. Similarly, major alien threats can be a source of new tek, evil alliances or simply outright destruction. In Black Crusade, at least, it doesn't take long for characters to rise to the heights of world- or sector-altering actions, and there is plenty in the Ark of Lost Souls to provide that sort of 'grand' opportunity.
I really liked it and the flexibility it gives. The layout is sensible and accessible for me, and the random generation system is actually quite fleshed-out, generating ideas that have depth and subtlety. As has been said, the whole thing is very much amenable to a complete campaign setting, offering challenges on all scales and for all parties. I am seriously thinking of having my players spend quite some time here, with the aim of somehow turning the resources and factions aboard toward their dark ends.
One particularly good feature of a space hulk is the opportunity to expose your party to xenos they might rarely meeet elsewhere, and this is well exploited by the material with plausible backgrounds. Finally, the chance to justify a Harlequin combat !
Good value, with loads of material and lots to stimulate your thinking. Even if you never plan to spend any time on a space hulk, there is so much here to give them more depth and flavour in your games.
Pleased I bought it, and am going back to read it again now !
5 stars would've been for slightly more surprises or twists, or moments of revelation, as my players like those a lot. Shouldn't stop you buying it though - a great resource, especially for anyone that loves the 40k setting.