What I like:

• The program doesn't cost anything, and after you get used to it it works quite well.

What I don't like:

• The developers ignored all guidelines for Mac programs: X-Lite uses its own GUI that absolutely doesn't fit into the Mac desktop.
• It uses its own address book instead of the OS X one that I would prefer.
• On my MacBook Pro the program directly crashes when a phone call is terminated. It also crashes when you just start and then terminate it.
• It took me some time to configure it until I was able to start my first phone call.
• If I want to use a Bluetooth headset, I have to activate it in the system preferences first (under audio settings, input and output) before I start X-Lite - I haven't found out how I can tell X-Lite to use a certain device for audio in- and output.

BTW: X-Lite is not a universal binary, but works more or less under Rosetta (see above). There's already a version 3 available for Windows, but not for Mac (yet).