
I've not been doing it for long - did it for a couple months last year, and am just about to get into it again. I do it for the dogs and for me - keeps all of use nice and fit (well, gets me fit first _then_ it'll keep me fit!!), and it gives the dogs something to do. I own a dobe and a rotti X, both working breeds (or mix of them anyhoo), so they need something to work at, to think about - and agility fits the bill. Plus, they are both very energetic dogs so it helps that too.
I find it helps with the bond I have with them - they have fun and so do I, and they get more responsive to me as well.
Makes me proud when they get the hang of another obstacle too :)