The first and third Halo novels are pretty good (written by the same guy) while the second one (tells the story in the first game) isn't that good, at least doesn't compare at all to actually playing through the game (was written by someone else too). I was pretty surprised at how good they were actually (but I dig the Halo universe).
The Diablo book was horrible.
The Resident Evil books (from the first second and third games) are alright. Not great, but not bad mindless reading. Haven't read any of the others, but they're probably about the same. They don't compare to playing through the games (which incidentily could probably be played through as fast as you can read through the books), but they're not bad.
Haven't read any others.
So, most of them are pretty good mindless reads, definitely not anything stupendous, but are pretty entertaining. Not bad diversions at least, and every now and again its good to read something that you don't have to think too much about.