I have a Cintiq 21UX and I love it. It's really expensive as you probably know, around $2,500.
I started on a Wacom Graphire, and then upgraded to an Intuos tablet and used that for years and years, and have recently (about a year ago) upgraded to the Cintiq. I really love the Cintiq, but to be perfectly honest with you, I don't see the productivity jump or the quality of work much improved from using the Intuos tablet. The Cintiq is without question MUCH more fun to use! But I think I might have gotten a little "too" good or too used to using the Intuos Tablet. I didn't find much that I could do on the Cintiq that I couldn't do on the Intuos tablet.
Bottom Line: I love the Cintiq, I don't regret buying one at all, I love using it every day. The only thing is that in my case, It wasn't as much different from using a Intuos tablet as I had heard/read/thought.
No regrets at all though!