The GM Kit provides an excellent introductory adventure for Deathwatch. It gives some additional information on the Watch Fortress Erioch and some key NPCs to flesh out your own depictions of this location. There is scant more detail on the Omega Vault, but it takes centre stage in the first scene - for me, I'm hungry for as much information on this device as I can get. It offers almost unlimited levels of creepiness for the 40K universe.
The module is great and pits you against a reasonable foe, with enough techno-supernatural blend to the mystery to mark it as keenly 40K. Te objectives are challenging, yet achievable, and the battles are frequent enough to allow the action to punctuate the drama nicely.
The last section on Mission Construction is very helpful, providing a wealth of advice about how all of the elements can be drawn together to create something greater than the sum of it's parts. There is a small among of discussion about theme, and my one complaint is that this was far too short. Ere needed to be a more fulsome segment on themes, given that the 40K universe has no immediate parallels with any other genre. Perhaps it has already Ben flagged for expansion in another supplement.
There are some neat tidbits of information scattered through the book too, such as stats for Landspeeders and Thunderhawks, as well as armament stats for both vehicles and some new Xenos equipment.
Just keep in mind that despite the extra info, this is a module, so the use is limited to th couple of times you're likely to use it. Given the recent reduction in price, I'd recommend it's purchase.
[4 of 5 Stars!]