Admittedly, once in a while I've picked up some adaptations that made me wish I'd left them on the shelf (I'm looking at you, Predator, Ghostbusters, and Ghostbusters 2!) but overall, a lot of the novelizations I've read over the years are reasonably entertaining. That's why I've never really had a problem with them or the authors who write/adapt them. Sure, they're not original works, but how many times through history have we seen authors rehash the stories told or written by others, and do it well? Plenty. The fact that these books come out on the heels of a story that's just been told on screen, and that they're a part of a studio's overarching marketing plan, doesn't matter much to me and borders on irrelevant. Now, I don't read novelizations all the time, in fact it's become a rare occurrence in the past 20 years, but that's more a statement about the size of the pile of original novels in my "to read" inbox and the amount of time I have available than anything else.
My top 5 picks for novelizations were:
Vonda N. McIntyre's adaptations of Star Trek II The Wrath of Khan and Star Trek III The Search for Spock
Orson Scott Card's novelization of The Abyss
Piers Anthony's take on Total Recall
Alan Dean Foster's (the king of the movie novelization) The Black Hole
Check out SF Signal to find out why these novelizations (and others!) are worth reading.