I was always a bit neutral about Billy Joel but we took the chance of seeing him at MSG the night after seeing Bruce in Philadelphia in February 2016. We went for the lower section behind the stage as we didn't want to spend too much and (after a bit of googling) we'd read a few reviews that said the seats were good.
I've never been behind the stage at any show before (it's not common in the UK) but we had a great time. At the lower level you feel really close to the action and as a piano player is 'side on' a lot of the time of there are no problems seeing Billy. In addition, his part of the stage rotates to provide various good viewing angles. In fact we loved the show and the seats so much that when we went back to New York in December we saw Billy again and sat in almost exactly the same spot and had another terrific time. The only thing you miss out on is the video screens but you are so close that doesn't really matter.
I'd have no hesitation sitting there again for Billy but you definitely want to be in the centre of the lower section around rows 12 to 18. If I remember correctly, if you are lower than that you may have some equipment at the back of the stage in your view.
Hope this helps.