This will be interesting. I've always been a pretty much make everything legal person, so medical marijuana sounded good to me.
However, a very good friend of mine has become somewhat famous in the fight against legalization in Colorado (not by choice).
He's a pain medicine specialist and has been fighting it in Colorado after seeing how things have turned out (he lives and practices medicine there).
I'll post a talk below he gave to a group fighting it being passed in Illinois. He's dead against it and from talking with him, he explained to me that the intention seems OK, but once you get it available then things take on a life of their own, especially with the drive to make weed more powerful. He said that very very few people should actually require weed (studies have generally shown that it's only really effective for a very limited number of medical conditions). After talking with him, I'm far less convinced that this is a good idea....
Go to the one hour mark if you want the summary.