I might jump from Skype to SIP if I had some more information. As it is, I'm not aware that I know any SIP users. Are there rating sites for SIP providers, apps, and hardware?
I'm using Callcentric and am very happy -- have also switched over my parents. I'm traveling the world and their call-forwarding feature (to my local SIM card in country X) works very well. http://www.callcentric.com/