The Downtowns are excellent as far as isolation and leakage are concerned. According to innerfidelity's measurements (http://www.innerfidelity.com/images/PhilipsCityscapeDowntown.pdf for the Downtowns and look for the other data sheets as needed), their 16dB broadband isolation is better than that of the Sennheiser Amperior (13dB), equal to that of the Sennheiser HD-25-I II (16dB), but worse than that of the Beyerdynamic ST1350 (19dB). These three are considered to be excellent isolating headphones. (Note, however, that the broadband measurement is a coarse measure that does not capture isolation at different frequencies.) The Downtown was also designed specifically to reduce leakage. See a discussion of this "MusicSeal" feature in Tyl Herstens's review at http://www.innerfidelity.com/content/philips-citiscape-downtown. If you need an isolating headphone and do not expect a neutral presentation, the Downtowns make a very good choice for the price. It is my favorite travel can. I can use it on a plane, and I do not miss my better headphones if I want to listen to 320kbs MP3 from a laptop or a phone for several days in a hotel room.
Thanks for the reply. I thought I'd updated this, but obviously I hadn't. I plumped for the Uptowns as they look bigger and sturdier. Hopefully they're just as good as the Downtowns at the above.