I have an iPod, and I love the Koss KSP-35 with it. Very portable, very good-sounding. A match made in heaven!
The KSP-35 sounds quite similar to the SportaPro or PortaPro phones -- they all use the same drivers. Most people on this forum think that of the three, the KSP-35 sounds the best. It is also probably the most comfortable.
I looked around for an even better-sounding pair of headphones. I bought the Sennheiser HD-497, and I found that they sounded almost identical to the KSP-35, but they hurt my ears. Then I went to the headphone.com exhibition in San Francisco, where I tried on about 15 pairs of audiophile 'phones. To my ears, the winner was the Sennheiser HD-580, which sounded a little bit better than the Koss KSC-35, but they cost >$200 and they are not exactly intended for portable use. So, I have decided to abandon my search. I'm thrilled with the KSP-35.