I sat in Row B when I saw Drowsy, and it would have been fine had I not been all the way on the side. As it was, some of the (fantastic) set wasn't visible, but almost all of the actual actors were completely visible at all times. If you are closer to the center, I doubt you'll have anything limited.
As for Spring Awakening, while I've never seen it from the mezzanine, I saw Sweeney Todd in the Eugene O'Neill from the very last row, in the corner, and had no trouble with it whatsoever. Everything was completely visible. And, considering how sparse the SA set is, I would think you'd be perfectly fine. The show might even be improved by your impaired view of the huge clouds of spit flying out of the otherwise adorable Jonathan Groff's mouth :)
"Why, I make more money than... than... than Calvin Coolidge! PUT TOGETHER!"
~Lina Lamont
My name wasn't, isn't, and will never be Scott.