One person I know was in the exact predicament as you are in right now.  He had that very same fantastic $30/month plan with Sprint.  You did not mention but his #30 Sprint plan also allowed for making the phone a hotspot so that you can tether other devices.  But when iPhone 4 came out, he fell into Steve Job's spell and succumbed into changing his plan to AT&T so that he can have his "Precious".

And he loves his "Precious" - iPhone that is.

But...

He regrets his decision.  

Dropped calls.  Slow speed.  Web page load crashes.  Tiered data plan.  Anything AT&T, you name it, he hates it.

He curses AT&T under his every breath.  If he sees AT&T maintenance van or truck, he has to physically restrain himself from flinging stones at it or driving into it.  He sold all his AT&T stocks and keeps badmouthing it in every analyst forum he is in (he works in finance industry in NYC).  One time he said he woke up wanting to put flaming bag with dog-poop in front of AT&T store where he got his plan, but fresh cup of coffee knocked some sense into him.

So there you are...