I went to three shows at MSG that were either behind or side stage, and at 3 different levels. Here are some thoughts:
10/8: Section 409, last row. This was more side stage, and we had a good angle, because we were right where there was a gap between the light curtains. But, being a 400 section, it was very high.
10/10: Section 218, last row. I had been giddy about these seats since the day I bought them. I was convinced that they were a steal. These were also kind of side stagish, though we were in the 2 seats farthest over in the section, heading towards rear stage. We were stuck w/ Adam being blocked out by a pole, and even though we were closer, they weren't nearly as good as I'd expected.
10/14: Section 321. These were directly behind the stage, and I enjoyed them the most of the three, I think. It was a neat vantage point, and the sound was really great.
I'd say go for it if you can snag some. I also saw a show in Boston from across the arena, and while I'm glad I did get to see one show from the front, I really enjoyed being closer and behind for the remaining shows. Oh, and yes, there are screens in the back and on the sides, though in the 218 seats, we were kind of between behind and side, so we couldn't really see the screens that night.