I'm not giving this four or five stars, but this is definitely one of the most FUN 40K books I've read so far, if nothing else.
Especially in the early days of 40K fiction, it seems many of the authors weren't QUITE sure how much of a satire the universe was, and how seriously it should be taken (the answer is, of course, 100% seriously, in order that the ridiculous satire of it be obvious). This resulted in many strange, almost experimental books such as this. It's been a few years, so bear with me, but we have basically three main plots, only two of which I remember.
Plot one is a Space Marine who's survived in space for a long, loooooong time. He was fighting some aliens & the only survivor at the end of the fight, his ship destroyed, so he just floated & many of the glands that are implanted during the SM transformation kept him alive. He "cocooned" pre-Heresy, in fact, and when the Chaos Space Marine who finds him & resuscitates him realizes this, he tells him, "Oh, yeah, we kicked the Emperor out long ago. Chaos rules now, bro!" Such a fun way to point out the Chaos/loyalist dichotomy.
Plot two involves a rogue trader and his crew venturing into the Eye of Terror for ... some reason ... and ... encountering things there. A veritable Alice in Wonderland ride of insanity, with crazy things appearing every dozen pages or so. A wild ride showing off (mostly) Tzeentchian Chaos followers, if I remember correctly.
Probably not the best for introducing someone to 40K, and definitely not the best book ever written, BUT if you're interested in going on a ride that won't let you feel safe, and want to be tossed into the world kicking and screaming, you couldn't do much worse.