Google Voice + Gmail Calling?

Internets! I need your help!
I use Gmail a lot, and just signed up for a Google Voice account, which has me thinking: I'm currently paying somewhere around $70-80 a month for my cell phone (which I use for home internet  as well), and I'm wondering if it would be worth dumping AT&T entirely and going over to Google, putting some of the money into getting internet at home (which is becoming increasingly needed) and getting a cheaper pay-as-you-go cell phone or jailbreaking my iPhone to do something similar.
A couple of questions for those who've used / done this sort of thing.
- How is Google Voice, and how do people use it? As a hub for any number of phones that you might have, or as a dedicated phone line? How's the quality?
- Can Google Voice be linked to Gmail?
- Can I concivebly jailbreak my phone and get a cheaper phone plan (small number of minutes, no data) with another phone company, or would it be worth buying a cheap pay-as-you-go phone and keep the iPhone as an iPod with a couple of apps on it? - Security-wise, what are the concerns?
Currently, I spend around a thousand a year on my phone. It's been a good investment for me since I got it about a year ago, but at the same time, if I can consolidate bills down to something much lower (or down to nothing, as Google Voice is free) that would be much more preferable, especially if I have wireless that can replace it? Given that my iPad and laptop would be a far more preferrable web browser than the tiny screen on my phone.
With the introduction of my iPad to the household, I've been using my phone differently - fewer games, more productivity and reminder applications (shopping lists, writing log, dictionary check book, etc, in addition to the text messages and phone calls that I get. The iPad has begun to handle most of the heavy duty writing (not that I did much writing on my phone), games, newspaper reading and when I can, web browsing.