We have it and love it! We haven't had cable in like four years (first had a bluray that streamed Netflix, then moved that to the den and put Roku in the living room so we can get current stuff).
You pay for the box once, but the "channels" (think of them like apps) you can stream are added from the channel store... some are free, some are subscription based.
We pay for Netflix and Hulu Plus, and have rabbit ears plugged into the tv (yes, they still exist, just plug into the cable input jack) and don't miss cable at all. We get all the network tv channels that way along with CW, Ion, Ion-Life, Qubo, PBS, and a couple of other randoms. Lots of games air on the major networks, so we watch those, and if a game isn't on, SO goes out with his friends to watch, so that works for us :)
The MLB channel is definitely on there, but I don't know the cost... I'm sure it's less than you cable package, though :).