It is very good. I'm not sure about the states but up here I don't think extra innings gets you all the games where mlb/tv pretty much does other thatn blackouts. The quality is very good and there isn't too much lag. I t all depends on your connection. Last year my roomate switched us to Cable light connection and MLB.tv wouldn't run worth a crap so you need normal cable connection at least.
I would recommend using Real player as well. I found that Windows media player was choppy but Real player works great.
. I like the fact that at night I can watch the 9th inning of about two or three games depending on timing.