This is a great soft Phone for Mac. Works with Asterisk and my USB handset just great. Also works fine with FreeWorldDialup and surely every other SIP service out there.
Supports multiple codecs (GSM, iLIBC, Speex, A-Law/U-Law) and I can even use it to talk to friends on dialup.
Every release I have used has been very stable. I used it with my iBook and free wireless internet access in a motel room to speak with my wife while away on business. It performed great and we had a 2 hour phone conversation no trouble.
The interface is ugly, but works great. X-Lite (another soft phone) is pretty and crashes all the time. This release finally adds a pretty icon, so perhaps a nicer interface is on it's way soon. :-)
SJPhone has been my soft phone of choice for a long time and I highly recommend it.
BTW, you can register for a free for personal use license number on SJLab's site.