I happened to have a pair of Sennheiser's PXC-350 headphones to compare them with - a slightly less expensive version of the PXC-450s, but nevertheless still quite pricey at around the £150 mark - and found the noise cancelling signal to be both more effective and less-intrusive on the Panasonic headphones. Flip the switch and you can hear very faint hiss, but it's not as loud as the hiss on the PXC-350s, and sound appears to be deadened a touch more effectively with the Panasonics too.
This isn't the only area where these Panasonic headphones match or better the Sennheisers, though. They're just as well made and comfortable to wear: if you're expecting plasticky build quality - and you might expect it for a pair of noise-cancellers under 120 quid - you're well wide of the mark. The earcups are wrapped in luxurious soft leather, as is the middle portion of the top of the headband, while the hinges and headband appear all to be made of hard-wearing metal.
Battery life is very impressive. The RP-HC500s take a single AAA battery, which slots into the left earcup, and this provides a quoted 40 hours of listening. Compared to the PXC-350s, which takes two AAAs and has the same quoted battery life that's a big bonus.
The RP-HC500s are also a lost less bulky and much more portable than the Sennheiser PXCs. They fold in a similar way, allowing you to stow them flat in the soft shell case provided with them and they come with a flight adaptor plug enabling you to plug them in while you're up in the air, but their lower profile makes you look a lot less of a git while listening to them, and the smaller circumference of the cups help achieve a snugger fit than with either of the bigger Sennheiser models. The downside to this is that to achieve this effect the small earcups won't fit everyone's ears though, and those with larger lugs will have to look elsewhere.
But it would be a great shame if you missed out on these because of your FA Cup handles, because the audio performance from these relatively cheap noise cancelling phones is pretty damned good. Compared to the PXC 350s, the clarity and balance isn't as good and the bass isn't as well defined. But they're a perfectly acceptable listen, and for just £115, arguably offer more for your money.
What I really liked about them was that they lacked the boxy, enclosed sound that so often afflicts closed back headphones. With the noise cancelling turned on there's almost an in-ear headphone sort of sound that comes out of the RP-HC500s, though you really don't want to switch the cancelling off. If you do, it sounds a bit like you're listening to music in a bucket ... at the bottom of the sea, with seaweed in your ears.