I think Watchers is a great introduction to Koontz; I think it's him at his best. If you like Watchers I think chances are you'll like a of his other books as well. I've always been very happy I started with that one - had I started with one of the ones I didn't like, I doubt I would have tried his other books at all.

I like the first four Barsoom books the best. In my mind that's "the first half" which is really good, and the rest are "the second half" which aren't as good. I feel like something changed starting with Chessmen but I can't be analytical about this series because the series as a whole is so etched on my brain that I can't quite pinpoint what it is.