Just how good is the 20/1.7?

Really?  How did you feel about the difference in AF speed? Have you noticed any other substantial differences?  And what about the half stop ev difference as well?

Also, my LR says I got about 12k shots right now, almost 9k are at 25mm (according to smart collection, little off due to duplicates), so if I were to go through with this, that 20mm is gonna be used a lot.