You might want to do a search of that topic on this forum. There have been quite a few threads with that topic and lots of opinions and reasons. Generally, it's considered one of the better OD's. It's not really one of the Tubescreamer clones though, and depending on which version, it tends to produce a lot of full-range response. If you're playing it through a Recto, for instance, you'll likely have to either dial way back on the amp's bass or use an EQ to tame the mud. That's often what I do using my v.4 through my Triple Rectifier.
The OCD can also sound pretty darn good all by itself for light to medium gain sounds, so one of my favorite places to use it is with the clean channel set to Pushed. Doing that can give me a great high-gain Marshall type of tone.