I have looked periodically at various IP programs, but I had specific needs.
1. needed to be able to port my existing number over. (very few do this)
Someone calling my US number, it rings to me anywhere in the world.
2) Voicemail access over the web with email alerts
3) Low cost and monthly service. (some have zero)
4) ability to use many devices like wireless PDA and use hardware I already had (IE: Computer, router etc)
5) Still be able to connect through analog phones
6) ability to forward Calls to my mobile
Everyone's needs are different of course.
After seeing sunrocket went bankrupt and the poor quality with Vonage, I have been using Call Centric.
http://www.callcentric.com/?aid=35113Cost me about $20 to port my existing number over and recurring costs are low, I can change the plan anytime.
I am only really using it to receive calls to voicemail and keep my old Home US number.
I still use Skype, MSN and other chat programs. (not true SIP or Voip) Used Iconnecthere in the past, but I didn't want to pay a setup fee and quality varied a lot.
One consideration is that because these all use internet the quality varies on your connection and the providers.
I have more than one, to ensure I can make calls when I want to.