Vi is a good Jungler because of her crazy ability to be tanky, as well as do a lot of damage. Vi is useful in team fights, and can chase very easily due to her Q and her Ult.

Pro's:

Very good ganks
Eliminates Katarina (a commonly used Mid-Laner)
Easy, non-dying jungle phase
Nice combos
Good stand-still combat
Eliminates hooks from supports
Good dragon stealing
Good fun!
Good late-game

Con's:

Shut down by CC
Noisy!
Requires mana for good use of Q early-game
Usually requires teamwork in 2v1's
Easily trapped
Not-so-good early-game
Bard. O.o

Don't max Q first. People in other guides say max Q, but don't. Your Q isn't very useful in the Jungle early-game, and your W will easily shut down the camps. Next go with your E. Your E should kill off small monsters in the jungle, and is very useful in an invade. Your Q, go with it last, because it is only useful for ganks, which at the earliest you will do at level 3, and you should have you Q, W and E by then.

You have a few choices, Q then E and finish it off with R is a good one if your opponent is running away. If your opponent thinks they can fight you, E then R and finish off with your Q, only if they try to run.

VI IS SO MUCH FUN!

Give her a go in your free time, and don't panic or rage if your team are dying early-game. You can easily carry the game back! You aren't as good as other champions early-game, but late-game your boss.