I use Koss SportaPros -- similar to the PortaPros, but less expensive. I use them with an old Creative Nomad Jukebox 2, and they sound great. The headphones are also good for long-term wear (I usually use them while out on 2 to 3 hour walks) because the foam bars above the earpieces take a lot of the pressure off your ears. I've been using them for at least 3 years, and I'm still happy with them.
I've never used the Sennheisers, so I can't make a direct comparison -- although I've certainly never heard anything bad about them!