When you're in a WiFI area, Line2 uses that for calls and texts. When you're in an area with data available, Line2 is smart enough to use the data connection for your calls. This app could save you around $50 a month on your bill, but even more if you drop down to the cheapest iPhone calling plan available.
The app itself is quite nice, with all the features of iPhone calling, including merge calls, speaker phone, mute. Plus, you can transfer calls or set up a conference call as easily as you can in Skype. Texting is nicely done with a great interface similar to Apple's. Voicemails are handled as you would expect and you can easily send them to others via email. Personally I like the implementation in Line2 of voicemail and calling features more than Apple's in some ways. The pop-up menu for emailing or forwarding a voicemail, for example, seems more intuitive than Apple's secondary menu.
If you're looking for a cheap way to make and receive calls, or you have issues with AT&T's coverage (and who doesn't?) you may want to give Line2 a shot. It just might be what you're looking for.