Almost all of my personal rides use 650c wheels. I'm a shorty at 5'4, and 700c frames in my size have gobs of toe overlap. 650c frames in my size have more reasonable proportions, so that's my sweet spot.

Having said that, there are problems. Tire selection is the most obvious, but don't forget tubes, as they are not as widely available as 700c. And if you need long valve 650c tubes for deep section rims - well, that's even more exotic. Not something a bike shop would necessarily stock, and almost all my wheels are deep section, so I can't cheap out and use a shorter valve stem tube.

Tires and tubes also tend to be more expensive, as they are produced in lesser amounts. And don't get me started on tire & rim combinations that won't work in any parallel universe, much less ours, where they are supposed to be the same size. Getting a pair of clincher tires on 650c Hed carbon trispokes was one of the most difficult tire mounting jobs I've ever done.

Bottom line: I wouldn't wade into 650c waters unless you absolutely have to.