It's a piece of equipment. Getting a stick won't automatically make you go from scrub to hero player. For me I had a good consistency on controller, but I wanted 99% consistency so I bought a stick to see if I could get that with it. At first I dipped to awful execution, before I slowly climbed back to where I was and then went to that goal of 99%.
The stick will most likely reset you, but it can pay off immensely once you stick to it.
A good stick, like the TE from MadCatz, will run you $99.99-$149.99. It was worth it for me, but it might not be for everyone if you don't play enough to justify the purchase. If your goal is to really up your game, and you're going to be playing plenty of MvC3 and other fighters in the future as well then it is totally worth it.