What's with the modern day setting. I thought this series was a fantasy Western.
There is a portal fantasy element, like Narnia, and a multiverse. Within the overall universe, sci-fi, fantasy, and horror intermingle freely.
In the epic fantasy genre, the series is one of the best ever. Obviously, there are a few problems as there will be with any series of this length, but overall the books are full of amazing situations that will stick with you for a long time. The first book is considered by many to be a weak point (but an interesting one) and some people don't like the way the series ends. The only book I had a serious problem with was Wolves of Calla, which came in the middle and had some pacing issues. But even that one had some great sequences and it was filled with all sorts of cool ideas.
I do like a good over-arching multiverse. The Shining obviously, and the monsters from The Mist being from the Todash Space described in Dark Tower.
Yep, this stuff is all over the Dark Tower books. Pretty much every Stephen King book ties into them in one way or another. Some of them, like IT and The Stand, are almost direct extensions of the Dark Tower series.