I migrated a medium-size webapp from Struts to Struts2.
If found Struts2 to be a very different framework (it is actually a merge-update of Struts and Webwork, with much more from the later).
I tend to like it, I consider it much more straighforward, elegant and easy to use than the old Struts (man, I'd hate to return to that one...). It is also relatively easy to learn, and there are a few good books.
On the downside: it's not easy to get confortable with the ONGL syntax + value stack thing, the documentation in the website is a little anemic...
And more important: the Struts2 project does not seem to enjoy very good health IMO. Not much depelopment buzz, little community. The future does not look so bright - other java web frameworks arent' much better, though...