I can only tell you my results from yesterday. I love the Wiifit, personally. People complain that a lot of the work-outs just aren't long enough, but if you do enough exercises you can unlock things like 10-minute runs or 6-minute hula-hoop work-outs. The yoga is excellent, as is the strength training. The balance games are a lot of fun.
I went out for a night of drinking and cookie-eating on Friday night, and the next morning my weight was 187.2. My family bought the Wii, I worked out on it for over two hours, and the next day I was 184.2. Obviously drinking beer at 1 in the morning would have inflated my weight, but I'm still thrilled.
It's great at keeping track of your progress and testing your balance. All of those yoga poses that pester you about keeping your core strength were always really confusing -- this one can actually measure it so you're doing your workouts properly.
I'm also pretty sore from yesterday, so it definitely is a work-out. I would recommend mixing up your activities -- start with yoga, move on to strength, then aerobics and then balance.