From: Little England
I make pretty heavy use of cavalry on all fronts bar the Western Front.
Against the Ottoman Empire it is pretty much a war winning weapon. The poor Turks just don't have enough troops to stop overrunning attempts by Entente cavalry. Note that the cavalry seems to work on the Caucasus rough at the moment - not sure if this is a bug or not! but in my last game I got the Russians up as far as one hex from Sinope with gratuitous use of cavalry and just two precious HQ activations. The British can have a go from Egypt along Gaza, though theres not as much space there so its not usually as impressive.
What I do is always, if theres a possibility of cavalry taking an enemy controlled square - always have a go. If you have three cavalry doing that on a sparse front like in the Ottoman Empire, then chances are one or two of your cavalry probes will actually be successful. Rinse, repeat.
You can do likewise on the Eastern Front, and in the Balkans too to an extent but not usually so successfully as against the Turks. But any hex captured is a Good Thing, so same again, just keep probing, keep on trying, every turn, it forces the enemy to make sure the whole front is garrisoned, and every hex you capture bloodlessly is a good thing, for the cost of the odd cavalry loss.