I don't know much about ankle holsters because I don't have a lot of interest in them. To be fair though, I have never tried it for any length of time.
I question whether its comfortable, although I suppose it would be with the right equipment and such.
My bigger problem is tactical. First, ankle holsters are slow. You don't need to be a quick draw to carry a gun, but the fact is that we carry defensive handguns to REACT to a situation that has alredy started, so there is some need to be able to act quickly.
Drawing from the ankle is also an awkward movement that requires going off balance to some degree or otherwise compromising your fighting platform -- again, by definition, after the fight has already started. It would also be difficult to do in cramped quarters or bad footing. Finally, access while running away from the threat is impossible.
Furthermore, there is no good way to draw discretely from an ankle holster or to get your hand on the grip in a firing position to be ready for a draw. When you carry IWB, you still telegraph something when you reach back behind your jacket--but at least there is some deniability that you were going for a cell phone or something. Pocket carry is great for this because you can have your hand on your gun in a firing grip ready to draw, yet look very casual and relaxed.
As you see -- I am not really a fan, especially for primary gun carry.
One advantage over pocket carry though is easier access to some degree while seated or in a car.
Hope that helps.