I've had the Wii/Wii Fit for a few years now and here's what I think about it. If you're already into video games then it's a good choice because it's definitely better than sitting down and doing nothing.
The Wii Fit specifically is a good first step to help encourage yourself off the couch. There are games to unlock on the WiiFit game itself, Free Step and Free Run are my favourite two which you can set up and then jog in place or step in place whilst watching tv except you're working towards a goal.
The only accessories I have are the extra WiiMote (so I can play against someone else), the boosted raised step and the 1-lb weights for the WiiMote. I wouldn't say they're necessary for the experience of Wii Fit though.
Regarding the intensity of the workout, I would consider the intensity, low to medium at best. If you're an absolute beginner, you may break a sweat and it's defintely, definitely better than being sedentary, but the Wii Fit games tend not to be tough workouts. Even games like Gold's Gym etc though more intense, are not exactly intense workouts.
For me, I use the Wii Fit when I'm looking to just get moving as opposed to lying on the couch. If I actually want a proper workout, I either go running or I do a workout video of some kind. I'm kind of a workout video-head and they're a cheap, good, at-home exercise option. I wouldn't get the Wii Fit just because you want to workout. I'd get it if you liked video games anyway and were looking for a way to stay fit and play at the same time.
: Jan 2009: 275lb; 3FC Start Weight
: July 19, 2009: 257lb; Record Low Weight
: Oct 21, 2012: 152lb
Losing the Regain- Jul-Dec 2014
Overall Weight Loss