Based on this thread, I decided to give them a try. I've registered a few dozen domains there since last month, and am liking it overall.
Like others have said, it would be better if they offered email forwarding. It would also be good to optionally have working, spammer-resistant email contacts in the private whois info rather than requiring use of a web form.
There are still a few minor bumps in their UX design. Every time I've asked support about one of these they have been great about clarifying things, and seem interested in making ongoing improvements.
And I wish the API also had JSON responses rather than only XML, but that's getting picky.
Update: I recently noticed that some of the UX changes I'd suggested a couple of weeks earlier have now been implemented. It might be nicer if they had an announcement board or blog to tell us about changes, but the important thing is that they do act on feedback.