I can't imagine switching away from Virgin Mobile, at least not to any of the big carriers. I was so happy to get out of my contract, and every time a friend renews their contract I just shake my head - especially if they have Sprint, because they use the same towers lol.
When I need a new phone, I may seriously look into Republic Wireless. One of my friends has it and is happy so far. It runs largely on WiFi as opposed to network towers, so it's significantly cheaper than even Virgin. Not going to go out and spend $300 on a new phone (they only have one, but it seems really nice) when mine is still functioning, but once it starts being a PITA...