I've had ST for over two years now and its great as far as I'm concerned. Depends on what you want. I love the $45/mo, where I was pay $150+. But, I don't need or want a $700 - $800 I phone. I have a $99 Blackberry, which works just fine for me. All I want is to be able to talk on the dam phone, send a text or two, and get on the internet to check the weather if necessary.
Most important thing with ST is to research and identify where you are going to be using the phone the most, as ST uses other peoples towers. For instance Verizon, At&t, and T-mobile as previously mentioned. You will want to select the phone and "sub-carrier" that coincides with where you are going to use the phone the most. Roaming sometimes is not an option.
If you are going to travel much, I would not go with ST.