I don't know too much about competition of Google Talk and IM apps on iOS, but http://imo.im has been good. One of Google's first 10 employees is behind the app. It seems to be the only iOS multi-IM app to support Skype text chat (including group text chat which is good for me).
imo.im is alright, but I found it to be a battery drain and to, for some strange reason, just straight-out refuses to stay disconnected (which is really annoying for stuff like Yahoo IM, which can only be signed in on one computer at a time). The latter is also a problem on Android, though I haven't seen the battery drain problems there.
I love it on the desktop, though - I try to avoid installing distractions on my writing netbook, so when I really need to talk to somebody I just go to imo instead of starting up an app.
http://verbs.im/