Haven't tried those ones either. Since the application has to be running to receive calls (no running in the background), I didn't really want to pay for such a limited feature - though I see iSip now claims to have push applications even when not running - not sure how this will work.
For the odd time I want to make a sip call, Nimbuzz has a built in sip client (it is a social networking / IM client / skype aggregator, but you don't have to use it for that). Seems to work well with 2talk, but I have not tested extensively.
I did not sign up to anything with either 12Connect or Weephone. It looks like the 12Connect product is connected to a service provider (called 12Connect), but they say on Appstore that you can use any SIP provider and I have been using it with my Asterisk box for a while now in addition to 2Talk.
I must admit that I prefer the look and feel of 12Connect and it connected with 2Talk even when my Asterisk box refused to connect just recently. So that is the one I have been using lately.
@mdf: I will try the oFone out. It looks like it could be a rebranding of another model in the appstore. There are quite a few produced by Mailvision i.e. IPico, Witel and oFone. I am not sure just what Push Notification is all about, but I wondered if it enables the SIP phone to be activated when its not the topmost client. If so, then that could be useful.
Edit: Looks like oFone needs an account setup with the OFone service. So not useful really as a general SIP client.
Most of the free ones, link to a provider. I just want a sip client. looks like 12 is the go, just waiting to get home and i'll test it with my iphone headset to see if it will use the mic. seem a lot of these sip apps are particular if they will work with a 3.5mm apple mic or not.