Basically a great phone. As a mobile phone, seems fine on O2. I bought it mainly to use as a Skype phone on the Internet without a PC.....I could not get it to work on Skype. I used it with Voip Buster, As Skype does NOT yet support SIP. Took a bit of patience setting it up, not for the faint hearted.... but when setup it works great.
Have a look at http://www.voipconfig.com/voipbuster_config.htm for some SIP config info with Voip Buster and other SIP servers.