I don't buy "adventure books" ever, between the price tags and the imaginarium inside my head, I stay away.
But something as iconic to the universe as a Space Hulk would be totally worth the buy, also the statblocks for things that could be used elsewhere add to the value for me.
Are genestealers already documented in the books? I only have the Core Rulebook and Errata right now and I haven't made it past the armoury chapter.
I'm have a similar mindset with published adventures - I prefer to come up with my own adventures - but I still buy them all. I view them, weirdly, as a MTG player might view a stack of booster packs: a chance to get new things to add to the game. In this case I mean new adversary profiles, and the first thing I do with an adventure book is flip to the NPC's & Adversaries section to see what we 'get' with this book.
Genestealers (and a lot of Tyranid units) are located in Mark of the Xenos. All the Tyranid units that weren't covered in Mark of the Xenos are covered in The Jericho Reach.