I recently tried the Sennheiser HMEC-45KA ANC open-backed headset. I found the ANC effect was good - all the "low level" rumble was taken away and I was left with a light hiss. RT clarity was excellent. I could easily communicate with the F/O and Cabin Crew without using the intercom.
However, I found the headband very uncomfortable on my (bald) head, and the combination of the earpads and my spectacles made my ears hurt! For £400 I would want something a bit more comfortable.
You can arrange a trial of the Sennheisers through Britannic Aviation in West Sussex if you're interested.
[ 18 July 2001: Message edited by: Chalky ]