Is Microsoft’s Ribbon UI really that great, from a usability perspective?

The first time I ever used it was at my current job. Among my co-workers, the feelings toward it for usability are mixed. The other developer doesn't really care one way or the other, as long as Office does everything he needs it to do when writing reports. The top manager likes it because it feels natural, and I feel the same way. But another coworker finds in clunky and hard to use (although she admits that she only uses it at home as her machine hasn't been upgraded yet, and that might change if she uses it more often at work).

• Is the Ribbon UI really that innovative?
• What qualities about it make it a good or bad user interface mechanism?

Possibly related: Adoption of the Ribbon UI