Cheaper Cell Phone Plan?

Has anyone used Straight Talk cellular and recommend them? Or other recommendations for cheap plans that support smartphones and unlimited data? Thanks.

Added: My phone doesn't work with the Straight Talk Plan, so I'd have to get another new or used phone. If you have had an IPhone 4s, would you tell me why it's great?

I Am Now Officially Old

I just joined AARP. I'm going to go with Consumer Cellular, and they offer an AARP discount.

Now I just need to figure out what phone to get. I'm trying to decide between an HTC One (probably used), an iPhone (probably used), or a cheaper model that probably won't work as well.