I'll admit I haven't read much of his work, but I enjoy his attitude. There is always a light at the end of the tunnel, no matter how dark it gets. And for those who say he's gotten a bit preachy with the Christianity, remember that he's been a Catholic for years. As far as I can tell it's been there through it all.
The books I've read: Watchers, Coldfire, Phantoms, Dragon's Tears (a bit over the top for me), The Key to Midnight, Strangers (real good), Breathless, 77 Shadow Street, What the Dark Knows, Darkfall, and I'm sure there's more but I just can't think of them so far.
He is a great author. I find his work almost as good as Tolkien's.