Let me say, I'm using AT&T data plan, Skype Out/In numbers, Google Voice, and iPad mini as my telephone. I sold my iPhone.
This all works surprisingly well, and I save over $60/month.
Here is what I don't like about the Skype app.
1. Can't use custom ringtones
2. Need to click through several alerts when an incoming call comes
3. When calling out, you need to click the contact, click voice, click the phone you want to call, then press the call button
4. When an incoming call comes in, I need to click the alert, then click the skype button to talk. I often talk before doing the latter.
5. Doesn't use Address Book images for incoming caller
And a few more things. If they could solve a couple issues, some are imposed by Apple, I'd love it.
Been almost one month with this setup as my main phone, and I love it.
Bryan