My experience with the clients and Gizmo / Google Voice.

Rtcomm / osso sip / internet call :
everytime I answer incoming, it disconnects.
everytime I make outgoing, it's hit or miss as to whether or not myself and the other party will be able to hear each other. (even when i leave inet call open).

Is there something I'm missing, I've heard leaving it open fixes problems, but for me its ~80% fail rate.

it would be preferable to use, however, considering I use it for my IM'ing.

Gizmo client:
Works very well. Never a problem. However, as stated, very heavy.

Fring
Never tried it, but planning on it.

///EDIT
Just installed Fring. I like it, except for the lack of DTMF dialing. Otherwise, it seems to be reliable.