I used Truphone for a year in my Nokia E65 through their free launch offer. Here's my take on it:
- exceptional voice quality compared to some other SIP services
- top notch customer service
- great, easy to use integration with Nokia devices
So why have I gone back to Skype on my Mac? Simple... their rates are some of the highest around! Just wasn't worth it. You can read about it in various threads here
http://www.mobilevoipforum.org/forumdisplay.php?f=49
Keep your eyes open for apps from Skype & iCall. But if you want to try Truphone, I really cannot complain at all about their service.