I subscribed to mlb.tv last season and have already paid for the 2011 season.
As others have stated the Roku was rough going the first half of the season but improved dramatically after that.
The mlb.tv package is available to stream on an ever increasing number of devices including phones and some internet connected blu-ray players and even some newer tv's, see http://mlb.mlb.com/mlb/subscriptions/index.jsp?product=mlbtv&affiliateId=mlbMENU
for more details.
I found the ps3 to be the best and most consistent platform for me from June when I purchased it to the end of the season plus the ps3 allows use of more features than the Roku. Towards the end of the season I found the Roku to be comparable to the ps3 in performance and watched approximately the last third of the season with the Roku instead of the ps3. The ps3 sucks electricity (about 160 watts) while the Roku sips it (about 8 watts) and so if performance, reliability and quality are the same I choose the Roku to watch.
I also subscribe to the nhl center ice season package and it is available on both the Roku and ps3 but I find for those games the Roku kicks the ps3's butt 95% of the time. I have used the Roku almost exclusively for nhl games all season.
I noticed that this season mlb.tv will be showing 150 spring training games live and wonder if Roku will be carrying those games. Only time will tell.
55" Insignia-Roku 4K TV, 40" Hisense Roku TV, Roku2 XS #13A164000326, R3 #1GJ36A038993, R4 wired to Netgear WND 3700 router, 300 mb TWC broadband (Subscriber- 2019 MLB.TV Premium, 2018-19 NHL.TV, Hulu+, Netflix, Amazon Prime)