On the one hand it's a much more approachable and user-friendly design (especially the preview feature), on the other hand it blocks valuable screen real-estate; this is especially an issue ever since the 16:9 trend took over the mainstream laptop and monitor markets.
The issue most users had at the time (and some still do to this day) was that they felt it unnecessary and obtrusive. Personally, I welcomed the change since the old menus weren't as discoverable and obvious to me.
Since the shortcut keys stayed compatible (I think?) and the ribbons could (and still can) be hidden by default, I never really understood the bad reception of this feature by many (old-school & power-)users.