I think it's a good example of a series that basically takes the sort of synopsis you'd expect to see in a movie and then expand it into a series or two, but the basic premise of the show - breaking out of a prison, and then trying to evade capture - only has so far it can go.
I'd agree that it had some good characters such as Bellick and T-Bag though. A prison drama is almost always going to be about characterisation and having a full series to work with gives a lot more scope to develop it than a movie does. Bellick is a good example, in most Prison dramas the guards are very predictable, either a hardnose baddie who bullies the inmates, or some sort of decent chap who tries to help them. Whereas Bellick develops throughout the show.