I don't use Sipdroid to make calls - but I use it to receive calls from a geographic number that I've set up via SIP.

I've found the quality to be pretty good - but I do only use it via wifi.

I signed up for a free geographic number from Sipgate, and set up sipdroid to automatically answer any calls that are made to it whenever I have connectivity.

I've found it's a fairly nice solution, as missed calls (e.g. when I don't have connectivity) are automatically taken care of by Sipgate's free answer service (and I'm sent the message as an mp3 via email).

I'm using it with an HTC Desire.