My thoughts:
1. Like any workout program, you have to stick to it and do it regularly for it to do anything.
2. It's not the highest level workout out there, certainly. It's not going to push you as hard as a 5 mile run, or a 30 day shred Jillian tape, or anything like that. There are a lot of pauses between exercises and most of the exercises aren't that hard. But that's not necessarily a bad thing, depending on your fitness level.
3. It is fun. I like to play and try to beat prior scores. That in itself I find motivating keep working.
4. If you've been sedentary to this point (or have any physical challenges), it's a great, gentle way to get in some movement every day. It can be a good jumping off point for getting started and eventually moving up to other things.
I use it, but not as my sole workout. I have other, more challenging wii games like EA active and some workout DVD's and I like to mix it up. I also walk or ride my bike outside some days, weather permitting. I use the wii fit plus on days when I'm not feeling that great, or on days when I don't do a harder workout but still want to get up and move a little bit. And, sometimes I get on it just to play and have fun.
I'm not sure you could lose a lot of weight just with the wii fit plus, but it can be a great way to get into exercising and to get a little more movement into every day.
I would also add that there is a new game out called "Exerbeat." I find it a little more challenging than wii fit, but it has the same sort of variety of exercises. It will use the balance board if you ahve one, but you don't have to have it. It's cheap, at $20. If the budget can stretch that far, I might buy the wii fit and toss in exerbeat for a little more variety.