I've DM'd two proper 4-hour sessions of Keep, and played a bunch of test encounters, so somewhere in the 11-20 range.
At the moment the biggest culture shock is the resilience of bad guys at low levels, causing most combats to become, at best, dangerous, and at worse, PK's waiting to happen. I think this is sinking into my players as well though. The larger kobold encounters in Keep are taking around an hour... not bad, but I expect them to become much quicker (and will be disappointed if they don't).
Overall the new systems are very easy to use, a lot of fun to narrate at the table (contrary to a lot of threads on this board), easy to retain in one's head, and an absolute breeze to prep. Only Stealth (shock!) has caused us any headaches, although ad-hoc rulings at the table are starting to coalesce into something approaching consistency now, and I think after anothe couple of sessions both the rogue and I will know what to expect from eachother. I have to confess to being a bit disappointed that they didn't nail those rules this time round.
With 2nd level around the corner, we're all excited to see the PC's develop through the campaign. Overall, I would have to declare 4E an almost unequivocal success... so far!