I have used it a fair few times to change from Sheffield to London trains onto the ivanhoe line (I prefer it to Loughborough because the southbound train can be held outside Loughborough to let the northbound depart) and I would say the car park is at best 75% full. It certainly looked more full the last time I was there (September) than it did when I first went there.
Always strikes me as a pretty decent station, odd that it is so desserted really.