I have two sip accounts. The first one was actually a gizmo account, but since it is using the sipphone.com network, I just entered the credentials in the built-in sip client and it works just fine. Incoming numbers cost a fee and the talk quality is miserable (at least to European countries). So I never use it.

The second one I opened at sipgate.de They gave me (for free) a number in Munich (where I live). It is quite easy to use it either on the N810 or on my E51. OTOH, sipgate insists on using standard ports, which means I can't use it via an UMTS connection (I can't be called, my phone network blocks that port). Here the quality is decent within Germany, and sometimes unusable when I call French numbers.

So in the end, I am not really pleased with SIP. Sure it is cheap, especially when I travel (compared to roaming charges or calling from an hotel room), but I would prefer a phone system that works.

OTOH, quality is fine between two sip accounts. I tried that when travelling to New Zealand, I set up another sip account and a phone adapter at home for that purpose at the time. The problem is that you need to have your friends or family use the same sip provider. And if you want that, you might as well get them another Nokia and use google talk. That's what I did with my mother and my sister and we now converse almost exclusively using the Internet. Quality is almost always fine, and we have the bonus of having the picture...