It's FAST too, in terms of calories burned per minute.
I love cycling - but any bike ride under an hour isn't really worth the time, unless you're doing hard intervals or climbing a really steep hill. If you have 30 minutes, there's nothing more efficient than running.
1) It's got a fairly flat learning curve.
2) The rewards are immediate and gradual, but clearly noticeable.
3) It's relatively cheap, though you can't cheap out on shoes if you are going to run on cement. A good pair usually costs over $100, but you can get them for half the price online if you know your size for a particular model.
4) It's convenient. However, you do need access to a shower afterwards.
5) You can do it alone or with someone else, at any time you want.
That being said, I found taichi to work just as well in clearing the brain, but you don't get the endorphin rush or the cardiovascular improvements as much.