Well put about when it doesn't. It can be tricky, and it definitely depends on how well the user can manage their vices. I'm able to, so it works out. I eat very well and exercise tons, so I'm very healthy otherwise. It's dangerous to exercise on the drug, which is just one of the many reasons I don't do it often.
I do know some people who smoke a lot of weed, and are super productive and sharper than ever on the high. Unfortunately, that's not me.