Some of his books are great. (I loved Odd Thomas.) Some of them stink.
He recycles character types too much for my tastes.
The same criticisms could be leveled at any prolific genre-riding author. I mean how many stories does King have about evil possessed cars and houses/structures that are actively evil? Eventually you hit a wall imaginatively and throw darts at the sticky-notes on the wall to find out what story you're going to write next to fulfill your publisher's demands.
And at least Koontz hasn't tried to cash in on a massive multiplayer crossover with his own stories. I think the closest he gets is using the idea of the escaped-the-lab super-intelligent pets in multiple books.
Twilight Eyes is easily my favorite Koontz book. The Christopher Snow stories are right behind those. Third place, probably Shadowfire.