Senn folders for a noisy world.
I have the PXC250s and they isolate pretty well in trains, jets, and on midtown Manhattan streets ... far from 100%, but enough so that you can hear the music. When you switch off the noise-reduction circuit (looks like a Boosteroo!) the background noise rushes in with a roar (rumble?).
They do not appreciably attenuate transients -- they mainly reduce steady sounds like Manhattan's background traffic noise, cruising jet roar, trains, fans/air conditioners, general low-level office chatter, etc.
I'll be upgrading to the PXC300 soon, but surprisingly Senn said they are currently out of stock! My 250s have a couple of external flaws and they're willing to replace with an upgrade (not free, unfortunately...tho' they really should, given the design flaws), so I'll wait for the upgrade to the 300s.
Don't try to analyze them in a quiet environment. The NR circuit imposes an audible hiss -- but their purpose is use in noisy environments, so why bother to compare them to high-end phones anyway? Their sound is enjoyable in portable situations, and unlike some who comment on their lack of bass, I find them quite satisfactory in that regard (see my notes on fit, below).
The ear pads are small, and anyone with large ears may not get an adequate seal, and so they'll get less isolation and a lot less bass energy. They work well for me, but I often have to shift them around a bit for optimum sound quality -- and I have somewhat small ears.
The 250s saved me from a major headache on a recent flight on which the pilot really gunned the engines for about 1.5 hours to get past a nasty storm --- the loudest commercial airline jet roar I'd ever experienced. Of course I still heard it, but without the noise reduction (and music cranked up) the roar was actually painful (people were shouting to be heard).
The one caveat for these phones is build quality. Although the headband and earpieces are surprisingly comfortable and durable enough, the belt clip is made of an unfortunately soft steel, rather than a superior spring steel as it should have been -- so that with frequent on-belt use the clip spreads open. I've found it necessary -- too often -- to use a big screwdriver as a fulcrum and hefty pliers to bend it back into usable shape. Also, the hinged plastic battery cover has an extremely flimsy plastic clasp which broke fairly quickly, despite using ultra-alkaline batteries, very rarely opening the cover, and generally handling it with respect. I related these issues to Senn Customer Service in writing and via phone ... very cheesy quality for a supposedly audiophile product.
They generally sell for about $149 at J&R in Manhattan, though I caught it on sale (in-store) for $99. Sometimes they're available on the Web for around $70 refurbished. I recommended this to a co-worker, and when he got them they looked new and in perfect condition. The PXC300 is about $50 more. (I have not yet heard the 300s.)
Those complaints aside, I like them a lot -- and I'm not a big Senn fan, except for the Orpheus system (...sighhhhhhh!).

BTW - not only am I also very reluctant to wear IEMs, but my ear canals are very narrow and twisty and so I find them very uncomfortable and irritating (at best). Even the smallest foamies don't fit adequately into my ears ... so to date the PXC250s (and soon the PXC300s) are the best bet for me, for portable noise reduction.