I love Wii Fit! I also bought the skin for the balance board, so I can just wipe off the skin to clean it. (Be sure to plug the plastic nubs on the skin into the screw holes on the board, it gives the skin a tight, secure fit.) It's not perfect, but it's an entertaining way to get real exercise--which I need. (Complaints: The trainers are very blah and generic, it would be fun if Wii Fit offered a "party mode", and people will have their own pet peeves.) The way it "banks" exercise minutes also encourages me to do "just one more".
I haven't even had it for two weeks yet, so I suppose my opinion could change, plus I'm someone with a LOT of weight to lose and a LOT of physical improvement needed. Maybe someone who's already really fit wouldn't find Wii Fit challenging enough after the first month; I don't know. My daughter is VERY fit and she still finds plenty of challenge, though.
The only problem I'm having so far is that Wii Fit has revealed how very weak my feet, ankle, and lower calf muscles are. I've cut my Yoga and Balance Game time in half because my feet and ankles were getting more and more sore, to the point where it was hurting to walk. Cutting the time in half seems to have solved the problem. Now those muscles get a workout and are a little sore, but not sore the next day. I'll be able to gradually add more Yoga and Balance Game time as those muscles get stronger.
Another factor in my buying the Wii Fit is that Rayman Raving Rabbids 3 will make use of the balance board, and I'll bet the only way people will be able to get the balance board is if they buy Wii Fit. At least 10 other games are in development to use the balance board.
So yes! I recommend Wii Fit if you can scrape up the money.
(That's me looking mighty after using Wii Fit.)