I already owned Wii Fit, but I bought Wii Fit Plus anyway, since it seemed like an easy upgrade.
Wii Fit Plus has all the games that Wii Fit had, of course, plus new ones.
I liked the new games, including the snowball fight one, where you can use the balance board to peek out from behind a barrier and then shoot a snowball. This type of gameplay subtly reminds me of the Time Crisis games, where you use a pedal to duck-and-reload while shooting at enemies with your gun:
The flapping game was cute, where you have to flap around like a bird to land on targets. The skateboarding games were fun, too.