Surprisingly good

Update: Wore these on the plane across country and they worked wonderfully. They're light, block noise, and I could sleep just fine. Careful with incorrectly grounded audio sources as the static charge may build up and shock you through the right earpiece! Only had a problem with one of my desktop computers.

I got these and a pair of Bose QC-2 headphones at the same time. This is in addition to the Bose QC-1 I have had for a couple years. It looks like the Bose QC-3 headphones are just like the QC-2's but have smaller earpieces to sit on the ear rather than over.

The Sennheiser is very nice and compact. The earpieces sit over the ear, but do not cover the earlobes. I thought this might hinder noise cancellation, however the noise cancellation is excellent, and is on par with the Bose headphones. The Bose headphones are a bit bulkier and cover the ears completely. Sometimes that is uncomfortable and gives me a slightly odd, queasy feeling. I also like to use them when I mow the lawn in the summer, so having full earpieces can get a little warm. This won't be a problem with the PXC300's. I can't wait to try these out this summer.

I love the fact that the PXC300 works even with the power turned off. This way I can use it without running down the 2AAA batteries. The battery pack uses two AAA batteries that sit on the end of the cord near the plug. The pack is compact and thin, however it's still there. This is a little annoying and an improvement over the battery pack hanging off the Bose QC-1's, but the Bose QC-2's have addressed this nicely by putting the batteries and power switches in the headset itself.

I think the sound cancellation is very much the same is a pair of Bose headphones. Price is very good, you could get two for the price of one Bose QC-2. I don't have a great ear for music, so I can't really comment on the frequency range.