Downsides? Well, it is streaming video, so it's somewhat pixellated (I watched a game at a friend's house in HD the other day, which made me think the mlb.tv stream is that much worse). The TV feed is always the home team's feed, so you get to listen to annoying out-of-town announcers (see the Tigers' thread last night for complaints about that). You can listen to the radio feed while watching the video, but they're out of synch, so it's somewhat of a pain. The video is roughly a pitch or so (maybe 45 seconds?) behind realtime, which means that if you follow the game threads you may get the time machine effect as someone posts about something you have yet to see.

For the cost, though, I'd say it's worth it if you're a big enough fan to want to watch the games. You can even just buy it month-to-month at $20 a pop (I think), if you want to try it out.

If you use a PC, it's not even a question - do it!

On the Mac, Mosaic works pretty well, but the generic stream you launch of the Gameday interface is not too good.  So I either go Mosaic or run Windows on my Mac when watching games.

I don't really care for what Mosaic offers, but the video streaming is done differently and better.

Give it a go....do it