I've been using X-Lite now for a week with BroadVoice as my VoIP provider. Overall, an excellent application.
The preferences UI could use some work, and it should really be integrated into AddressBook. Thankfully, there are AppleScripts to make a half-hearted attempt at integration available.
For the price (free), this application is well worth it. I think they should consider including a settings URL import option that allows you to import settings from your provider, rather than having the user input all of the confusing configuration options.