Pros: Naturally sounding, very comfortable
I purchased these because my Sennheiser PXC450 were getting too big to carry around once I purchased a larger netbook. So this review is more a comparison of it and the QC15.
The striking difference is the size - the PXC450 are just massive. The QC15 on the other hand have cups that just fit around the ears, and are lighter. The sound quality on both are excellent, but my personal opinion is the QC15 produce a much more natural sound. I have also tried a friends Goldring NS1000 (which is a lot cheaper) and while its noise cancelling is good, the sound is a lot more punchy than that of the PXC450 and the QC15.
The PXC450 and QC15 both come in a carry case which does its job, and have a handful of helpful accessoires - airline plug adapter, compartments to put extra batteries). One thing I do not like is that Bose puts in 'business cards'. Not only do I find this smug, but I'd rather they dont do this and reduce the price of the headphones. I ended up taking the Bose cards out and putting my own cards in.
I only use the QC15 during my frequent travels, so my review emphasizes more on the noise cancelling aspect. As a frequent traveller in Europe and Asia, I get to fly on a variety of aircraft types, and here is a list (of the most common types) of what I think:
Boeing 777-200ER (rolls royce trent engine - and old one so rather loud):
PXC450 - whilst it does cancel out a lot of noise, the low freq engine rumble is still noticeable - although in a very reduced manner. The large cups makes them very comfortable over long haul.
QC15 - similar to above, it does cancel out a lot of background noise, but engine noise is still noticeable. The sound seems to be more natural than the PXC450.
Panasonic RP-HC55E-S - I have a pair of these in-ear noise cancelling earphones, and they do cancel out more engine noise than the other two. However, there is a design flaw on these earphones that I really would not recommend you buying these.
Boeing 747-400 (P & W engines - old and loud, even on newer models of the 747)
PXC-450 & QC15 - the background noise is determined by seat position. The closer you are to the nose/tail of the aircraft, the less background noise is present (in my opinion). Both headphones perform at similar levels in terms of noise cancelling with QC15 having an edge in natural sounds. Over extended (10+ hours) flights I prefer the comfort of the QC15 over the PXC450.
Boeing 737-700 (CFM engines)
PXC-450 & QC15 - both do a decent job of cancelling out noise.
Airbus A312 and A320 (CFM engines)
These newer types of aircraft use 'quieter' engines (according to brochures etc). While this may be true, this does not seem to matter much when it comes to the PXC450 or the QC15. Both do an excellent job of cancelling out noise, but you can still hear (low frequency) engine rumblings in the background.
I think these are the most common aircraft types. I've flown with many more (such as small regional carriers Embraer series aircraft) if anyone is interested in what I think of them.
To compare the PXC450 and the QC15, they both do a good job of noise cancelling. I'd give the edge on the QC15's in terms of sound quality.