I think it depends what you're expecting of it.
Ds1 has ASD, hypotonia, some hypermobility, fine and gross motor issues and poor proprioception.
Ds2 has joint hypermobility syndrome, hypotonia and some issues around gross and fine motor issues and proprioception.
They had a Wii for Christmas.
In reality, a lot of the more popular games don't actually require them to move much - they can easily sit on the sofa and drive their mario kartz, for example.
You can however be selective and we have found that they move a lot more when playing Wii Sports, Wii Party and Guinness World Records. You can of course also buy the Wii Fit kit, which we plan to do sometime soonish and can buy games that incorporate the Wii Fit wobble board, such as ski, snowboard and skateboarding games, which are handy for balance practice.
I don't think the Wii could ever be described as 'exercise' exactly, unless you are concertedly doing the Wii Fit exercises, but I am impressed at how much my ds's control of their 'controllers' on the screen has improved since we got ours and do think it's helping, at least in some small way with at least their spatial awareness. I also prefer to see them on their feet waving their arms around than curled up in a ball or lying on their beds playing on their Nintendo DSi's.
It's never going to replace or even come close to a proper OT/exercise plan, but in terms of motivating a demotivated, struggling child to at least get up off the sofa and move themselves around a bit - yes, imho it's probably been worth it for my family.