Wii Fit Plus is great, it's a big improvement over Wii Fit.
It adds a bunch of great new games to the what's already there, which are great if you have started to get bored of the ones in the original.
More importantly it adds a routine system which makes working out with it much easier and more effective. You can programme routines in and store them for later (I have the entire yoga set programmed in), or you can have it build you a routine based on areas such as your tummy, or legs.
Just to clarify two things:
- If you already have the Wii Fit, you only need the upgrade disk, you do not need to get a new balance board, that has not changed. It's about £15 for the disk.
- Wii Fit Plus has nothing to do with the Wii Motion Plus controllers, and works just fine without them. I think that was a bad naming decision on Nintendo's part as it has confused a lot of people.
I've done a pretty in depth review of Wii Fit Plus at WiiFitForum.org* (in the "Reviews" section) which you might find useful.
I hope that helps. Personally I've found that upgrading has totally revamped Wii Fit for me, I'd got pretty bored of the games, and annoyed at the lack of a routine system like you get in EA Sports Active. It was totally worth the extra money.
Awesome site by the way, I wish I'd found this place earlier
* That isn't meant as spamming but feel free to remove the link if it's against the rules, I wasn't sure.