I suppose it depends what your expectations are.
My experience is that you can see the ball, and seamlessly follow the game, from 'bad' seats in the corners of the upper tier (though I have never sat right at the very top). You're right, though, that you probably won't be able to read players' facial reactions, though you can read their general body language and gesticulations.
I hadn't watched any games in what I would have considered to be 'bad' seats until last year, when I picked up some cheap tickets at the last minute for sessions I wanted to watch, as I had the same prejudices about the view. I was perfectly happy following matches from those positions, though, so I'm now fairly indifferent as to where I sit.
If you want the up-close tennis match experience of seeing every ball zip right by you and every wince or groan, then buy lower tier tickets. If you are satisfied in just following the game itself, and sucking in a bit of the atmosphere, then the upper tier is fine (though again, the lower rows of the upper tier will feel closer than up at the very top).
Also note that the schedule of the tournament has changed this year, so it runs from Monday to Monday, rather than Sunday to Sunday. This means that within the group stages, the best bet for watching the top four play each other would be the Wednesday and Thursday (as winners of the first matches play each other).