A few small commercial gyms have implemented this, as well as the gym at Brown University. Brown estimates that the power generated by its 50 machines is about comparable to what's generated by the solar panels on the roof of the gym. So it's not nothing, but gyms tend to be cavernous and expensive to run, so it's a drop in the bucket, unfortunately.

More unfortunate still is that the takeaway is almost always, "ugh don't bother, it's basically nothing" and not, "wait, why are we burning so much fuel to power this room?"