I have gizmo5 (no new signups possible) and sipgate. On wifi both are very capable once the call is established. Gizmo behaves flaky every once in a while. Sipgate is much better in that respect. Skype also works, but its useless if you want to send DTMF tones (touch tones).

Both work well on 3G - when that 3G actually _is_ 3G. What I mean by this is using 3G frequencies does not automatically imply better speeds. Tmobile (or any other telco) seems to have many pockets in which 3G signal is superb, yet the speeds are dismal. I checked this with speedtest as well data transfer rates over my personal vpn.
What I found was that if the 3G data rate of a specific location is good, then voip calls are good (well duh!).
For bad rates, sipgate and gizmo were about equally flaky, but Skype was flakier.
In 2G / 2.5G, sipgate was the only one that actually worked well enough to have a decent conversation - but that was just one time. In every other place where I have attempted a voip call over 2/2.5 G, both parties have failed to hear anything.

I also found that any other process that eats into the n900 processor will cause weird (interesting) audio artifacts in a voip call. This includes the web browsing, getting another call or text while talking, playing Angry Birds...
Opera browser is more of a processor hog than MicroB and so attempting to browse in Opera while on a voip call is the best way to shorten the conversation and battery life.