if cross compatibility with iPhone and such is a big priority, Tango is where it's at. it works very well over 3G even. i have yet to try Fring, but i've read good things about it. i thought Qik was okay too, but is basically android to android, as far as i know.
i really like the Qik app for Evo though. i use it all the time to share private videos with my girlfriend. i hope their video chat gets a little better, and becomes cross platform, because as i said, i really like the app in general. i think they're on to something.
lol, not private videos like that. just of me saying hi and looking all handsome and ****.