Recently I have been forced to use my old x800gto. Open source drivers work marvelously. For stuff that the regular user does on a daily basis they're more than enough. Movies, DE effects, games (most of then, and most certainly not WINE games), all function reasonably well. The advances being worked on will only make them better (3D speed, more complete support for newer chips, etc.). I can imagine most users or pre r600 cards being very happy with open source drivers.
For me the only real advantage of fglrx is 3D optimization. Speed in 3D applications is always a good thing, and the more you have the better, and if this is of importance to you chances are you have a rather new card, which are in turn supported by fglrx. I really don't see what the big fuss is about, specially on the Linux side.