Ankle holsters are hard to draw from in a standing position. BUT, they are really good for seated positions, like in a car. Whereas a pocket holster is all but impossible to draw from while in a car, an ankle holster is quite comfortable and readily available. Just remember to wear pants long enough to cover the holster. Nothing like sitting down in a public restaurant, and having someone ask why you have a gun strapped to your ankle.
From standing, the best way to draw is to kneel on the opposite knee from where the holster is, keeping your eyes on the target. Pull your pants leg up a mite with your off hand before doing this will make the draw easier.
I use a Gould and Goodrich ankle holster for my 640-1 and my 642 airweight. I mostly use them when traveling, so the gun will be available without being left in the vehicle.