I use different free and paid softphones for my voip accounts. Based on my own experience, I can draw some advantages of iSoftPhone over similar products:

1. Very clean and useful integration with Apple Address Book. So there is no need to re-enter your contact and keep updating them.
2. In case of using more than one SIP account, it is very easy to select your desired account for making calls. You can not find such an easy feature in other similar products.
3. Very exciting skin ! (like my iPhone)
4. Very clean sip account configuration page.
5. Faster account logins.
...
end now with even more competitive price of $35
I will try to provide more on this later.
Thanks.