If you have read anything else by Dan Abnett, you will already know the quality of his writing. If you have never picked up a Dan Abnett novel, this is as a good place to start as any. I would whole-heartedly recommend this book to both fans of Warhammer 40K and people wanting to get into the books. There is nothing extremely graphic, but there is a fair bit of violence and the odd swear word. However, these things do not feel shoe horned in just because it could be but rather add another layer to the worlds and make them more believable.
The Writing Itself -
His writing is rich and his descriptions vivid and well crafted. You cannot help but be submersed in the world of Warhammer 40K, the lives of normal people away from the frontlines. The characters in Eisenhorn are varied, realistic and non-cliched - You will care for these characters. The plot throughout was crafted well and there were enough twists and turns in it so you are not figuring everything out half way through.
Favourite Thing? -
The thing I liked best about Eisenhorn was that it took you away from the front lines and to planets where battle has never been seen. The book gives you the view of a "normal" person's environment and everyday life, even though Eisenhorn is doing his job as an Inquistor from the Ordos Xenos. Also, Dan Abnett is able to keep you hooked and turning page after page. He builds suspense very well too.
Worst Thing? -
In my opinion, the ending felt a little rushed and there were several loose ends. The worst thing about it overall was that it ended. I got to the end and wanted more of Eisenhorn's character because he was so well made.
Also on Ciao! under the same username :)