Conclusion: For $260, Sennheiser's PXC 250-II is a good pair of noise-canceling headphones, but if you can afford them, more expensive pairs, like Bose's QuietComfort 3, deliver much better performance.
Pros: Effective noise cancellation. Can be used in passive, non-noise canceling mode. Solid audio performance at moderate volume levels.
Cons: Distorts at higher volume levels on bass-heavy tracks. Large noise cancellation/battery compartment on cable. Runs on a standard (not rechargeable) AA battery.