I am in the last stages of PADI DM right now, packing for a 2 days of OW checkout dives as a matter of fact. Unless you intend to actually be a DM or instructor, frankly it's a waste of a lot of money and time. A lot of the "skills" you learn as a DMC are a good bit different than you actually use in the real world. For example, it takes me about 8 seperate steps and 45 to 60 seconds to demo flooding and clearing a mask. Real world, 2 or 3 steps and 5 to 10 seconds. The skills you aquire are geared toward teaching people how to do the skill, not how to do it real world. You learn how to demo it well to non divers (students), not fast and efficiently - totally different goals. Or how to do a perfect fin pivot, lots of use for that. The hardest thing for me has not been the skills themselves but learning to sloooooow down and break things into steps. I now find I have to alternate between 2 ways of doing many of the basic skills depending on if a student is around or not.
On the other hand, you do learn quite a bit more dive physics and biology but if you are interested in that, buy the "Encylopedia of Recreational Diving" and read it yourself, no real reason to take the test. Your awareness and buoyancy skills do improve but that's more due to the time you spend in the water and watching the students than any instructuction. I don't have any plans to become an instructor but I do have to admit I have enjoyed working with the students and will most likely continue to DM for some of the instructors.
If you want to really learn new skills or improve on your current ones, I would suggest DIR-F and cavern as a couple of good choices. I don't care for a lot of the DIR BS but there is no question in my mind that the DIR-F is one of the best courses for improving your basic skills like buoyancy, trim, buddy awareness and gas management. DIR_F and rescue are the best 2 courses I have taken to learn how to be better diver.