We did a test run of Keep on the Shadowfell last weekend (continuing it tomorrow). I was very much against 4th edition but I had a lot of fun playing it, and I'm actually looking forward to tomorrow's continuation.
I disagree with the notion that clerics are just buff bots who are light on combat... the cleric has plenty of combat capability, and only had to buff one time. In fact, I'd say of all the classes we played with, cleric is still the most well-rounded.
We're going to try to push through to the end of Keep, but so far things seem very positive. Yes, it is fairly simplified, but I don't consider that a bad thing. I never liked games I had to "study" for.