I ONLY use my N900 as a Skype in/out handset. No voice/texting plan on it, only data. I haven't tried it in a while, but I think the only limitation I had was I couldn't send DTMF tones? Maybe this is fixed and I just don't know about it though, I've honestly not tried since I found out the phone doesn't support doing that.

Wish Skype had video support though.