I usually have to watch the 800k stream, as the 1.2 gets a bit choppy for me, but I'm on a laptop, 1.7GHz with only 512Mb RAM, but it still looks good. I usually watch the games the next morning--I drop my wife off at the train station at 6am, and then come home and watch the ballgame (I "work" from home), so there's likely not much traffic on the MLB servers. It's kind of weird, though, that at 6am my time there's usually one or two West Coast games still going on, so I caught some of the Astros/Giants games live, which is a rarity.
But, all in all, I'm very happy with MLB TV.