I purchased a Bose X headset earlier in the year, having used Dave Clark's for many years. The difference in performance is dramatic, it is without doubt the best headset I've ever used.
The noise-cancelling performance when you turn on the battry pack has to be experienced to be believed. They are also extreamly light to wear and therefore comfortable for longer trips.
There're expensive, but what cost do you place on your hearing. Best purchase I've made in a long time.
I bought mine from a well know retailer north of the border as I've always had excellent customer service from them and I know if I have any problems with them, there won't be an issue in replacing or sorting out the problem - something worth factoring-in to a circa £700 purchase. Hope that helps.