I think it works great. You can have completely free calls anywhere in the USA by getting an sipgate number, set google voice to forward calls to sipgate. Setup a pbxes.org account which forwards incoming calls to sipsorcery and a sipsorcery account that will automate the making the phone connection for google voice to connect the call for you any seamlessly route your call back to you sipgate number.
I'll admit it's a pain in the butt to setup, but it will allow you to make and receive calls for free over data plan or wifi.
ipkall is another site you can use to get a free DID phone number (although it must be used with something like pbxes.org)
the only part i'm not sure of, is whether sipsorcery is still excepting new accounts.
yeah, i just checked. they periodically are accepting new accounts and post them here:
http://sipsorcery.wordpress.com/new-accounts/
Anyway, I don't really have the time to go over a step by step guide but there are many out there if people are interested.