I went from -15 W/L to +25 W/L on my normal account just by jungling hardcore with Udyr and Yi. Honestly, though, jungling has its own skillset associated with it, and while it doesn't revolve around reflexes like last-hitting, it DOES revolve around prudence and judgment.
Let's consider a situation for a second: I'm playing Udyr, and you're playing Amumu. Udyr is a far stronger jungler than Amumu. So to exert my dominance over you, I'm going to invade you at Red to try to kill you (I probably wouldn't do something this risky in a real game, but just for example).
But what happens is that my mid is at half health, and your mid is at full health. So as Amumu, you call your Ashe to help, and she comes. I call my Karthus, and he comes. Except the full health Ashe beats my poor Karthus.
And the sad thing is that Karthus' death is entirely my fault for invading when HE was weak.