I installed the update only a day after getting my 990. I can't speak to the relative sound improvements since I hadn't listened to it much prior to the update. However, I very much appreciate the surround mode button behavior change. I find myself checking this quite often and if I had to cycle through the modes every time, it would have been irritating...

FWIW, I updated via USB and it was painless. So yes, I think it was definitely worth it.