I have been really impressed with the writers I have been exposed to from the Black Library. Dan Abnett and Graham McNiel are really top rate. I would go as far as to say that Dan Abnett ranks up there with some of the top military Sci-Fi authors that I have ever read. For sure he is comparable to John Scalzi, Orson Scott Card and Heinlein. Graham is no slouch either and with the Fulgrim book he is really top rate.
At first I was really a Sci-Fi snob when it game to the 40k books since I have always considered series books like these that are based on Games, TV shows or whatever to be cheese. But the Eisenhorn book and then the rest I have devoured has really changed my mind. I came to playing 40k pretty late but with the combination of a fun miniature game and a great universe that is being built up constantly by these authors it has me hooked.
Now if only GW would really start driving their mini and codex releases based more around these books that would be awesome. :)