Handshakes just seemed rather clunky to me - phpfox is a lot more slick. Granted, it's got a few issues, but I find it better than Handshakes in almost every way. phpfox seems to have a much better community behind it as well, and support is much easier to come by.
A big thing (at least for me) is that phpfox seems a whole lot more like MySpace. That might be a turn-off for some, but it's what I wanted.
As for users, I had to just start over. The site was only a few days old, but I lost 400 users because of the switch. I e-mailed them and explained that we were switching and most of them signed back up on the new site. If you have more established site, switching would be a bit more difficult.