Skype + Philips VOIP644 + GrandCentral + Prepaid AT&T Cellphone for the last 6 months. No complaints here. The only catch you may have is getting a "local" phone number from Skype - which is why I use GrandCentral.
One other thing to note, some places won't take your call or even ring through because the Skype out calls show up on caller ID as a weird number (Private - 0123456789) - for some automated systems, they just can't cope (had this with JetBlue a couple weeks ago, couldn't get through from home, but worked fine at work).