I've been using the Wii fit, and more recently Wii fit plus, since they came out, and I LOVE them!
I also toyed with a gym membership, but it's just so much hassle compared to doing it indoors, and as far as weight loss goes I only really need the cardio vascular stuff which is in abundance on the Wii fit games (in particular: free step, free jog, rhythm boxing, obstacle course, cycling, and I couldn't leave out flying like a chicken haha). All good fun, all in the privacy of your own home, all without having to travel, and all free once you have the Wii and game!
As for the weight, mine has always been quite accurate - about 2lb different from my actual scales. If you are doing it to help with weight loss though, it doesn't matter how innacurate they are, as however out they are for your initial weigh in, they should be the same amount out for the following weigh ins. In other words, despite the actual weight showing wrong, the weight loss will be correct. You have to make sure you place the board in exactly the same place each time though.
If you have a thick pile carpet, you can get leg extensions for the fit board which help with accuracy.
I have recently started to put a block of wood under mine, lifting it off the floor a further 1.5 inches or so. Weights not affected, but it makes me work that little bit harder each time and I suspect is helping my weight loss/muscle development a little more.
I also love My Fitness Trainer Cardio Workout, which is basically a boxing training game. You don't actually get to fight anyone as such, but it is about the training a boxer would have and it works me into much more of a sweat than Wii fit ever has!
Have fun