Only about 40 batches so far and no dumpers yet, and really only a couple of batches I was unhappy with. I am fortunate enough to have started brewing after the internet made so much information readily available and the brewing explosion made so many excellent ingredients readily available.
Closest I had to a dumper was an ESB that smelled like sulfur every time I brought the pint to my lips. My friend thought it was ok, so I left the keg at his house for him. He said it actually got much better with a little age.