I gotta reiterate here - if isolation is an issue, and if you would still like to have a noise cancelling function (great in planes, trains and automobiles), then open 'phones like the ATH-AD series are simply NOT what you're looking for. They may sound wonderful tested against comparatively-priced closed cans, but you'll hear everything going on outside, and everyone will hear what you're listening to.
The ATH-ANC7 is very comfortable, isolates well, and sounds exceptionally good for a noise-cancelling headphone in it's retail price range (~$200). The fact that it is actually going for around $130 and comes with a nice semi-hard case and cool accessories like an airplane-specific adapter makes it a steal of a buy.
If noise cancelling is no biggie, spend you're money on closed versions of the cans everyone is recommending, like the ATH-A900. Audio Technica 'phones are a good place to start for their relatively balanced yet 'fun' sound.
I own the RP-21's and heartily recommend them, but with $150 you could probably get something a little better.