I wasn't a big fan of my seat location. I was in the B section of row 26 on the Main Floor. The music was a bit dim sounding from back there. Also, because there isn't a steep decline descending to the stage, you're forced to watch through a thicket of heads. If you're contemplating buying Main Floor tickets, I wouldn't bother with anything above row 14.
All that being said, Symphony Hall really is very beautiful--both in the hall and the lobby areas. I saw the Nutcracker here recently, and the quality of the production was great. The sets, music, and dancing were all top notch. I'd like to return for more productions.