Decent.
The graphics/player animations were pretty good, and the idea was fine, but the controls lacked a bit.
When you jump you can't even swing, so how can you hit the higher balls? At first I could never make a homerun, but then I got the hang of it and it came easier. Unfortunately, sometimes it easier to hit the ball by simply holding the spacebar and standing still, which ruins the element of skill in the game.
The "HOMERUN!" announcement was kind of annoying, and the quality of sound wasn't very good.
You should have made it so that the music continues when you go from Training back to Menu. Also, you should have looped it so that when the song finishes, there isn't just silence. If not that, at least a Play/Stop button for music.
Overall a decent game, but nothing spectacular. There is lots of room for improvement.
P.S. I'd just like to mention that you used my song "in the meadow" without crediting me. I've sent you a PM, and I hope that you give me proper recognition soon :)