tl;dr: Solid performers for those wanting a bass-heavy aural experience and good isolation. The headphones can bring new life to some songs, but are often overbearing for those with sensitive ears.
Finding a good pair of headphones can be an lifelong vocation. In my short lifespan, I've probably given a good chance to about twelve different pairs, from earbuds to large cans used in professional/studio environments.
The holy grail is a pair of earbuds that are lightweight, comfortable, and give at least 85% of the aural experience of a large set of high-powered cans, for typical listening/music scenarios. Things like a nice design are optional.
In one word, are the Sony MDR-XB40EX earbuds this holy grail? No. But they're darn close, especially considering the price!
In the box, you'll see the default/medium-size pair of ear canal inserts already installed on the earbuds. There are two other sizes (small and large) hidden in the box, but for my ears, the mediums were perfect.
Coming from open-ear headphones to typical earbuds is a somewhat unnerving transition—instead of having a nice air gap between the sound driver and your ear canal, the driver is stuck inside your ear. Going from earbuds to an in-ear headphone is similarly unnerving. These in-canal phones are no different—there's a bit of a dreamy sensation you can get while wearing them, where you feel like you're swimming in your head.
However, this sensation is easy to get used to, and after wearing them a few times, your ears will adapt.
Unfortunately, though, for many people, their ears won't endure this feeling for more than an hour or two (include me in that list). Depending on your listening habits, this may or may not be an issue. If you only listen to music sporadically, or watch a single movie on your iPhone or iPad, it's no problem.
The headphones are easy to insert into your ears, and they remain firmly in, as long as you use the appropriate size insert. be careful to not yank on the cord, though, as this can cause a little pain in your ear canal.
The XB40EX's 'EX' stands for 'extra bass,' and these phones live up to that nomenclature. All music, whether classical, rock, hip hop, or techno, is infused with a lot of bass. For well-recorded audio, this is actually a good trait, in my mind. I love a saturated low-end—especially in orchestral pieces with a large bass section (1812 Overture comes to mind).
However, for a lot of modern music, especially pop and rock songs, your ears will start hurting unless they're already damaged, if you listen to music for more than an hour. Listening to songs with already over-saturated bass beats (like OK Go's This Too Shall Pass) made my head hurt almost immediately.
The rest of the range is pretty well-represented. I would compare the experience of listening to John Williams' Star Wars soundtrack on these headphones to listening on my much larger utilitarian cans, the Audio-Technica ATH-M30 Headphones [Amazon.com referral link]. It's a much better experience than listening on my stock iPhone/Apple headphones.
One added bonus is the ability for these phones to isolate you from the surrounding environment. One reviewer stated that he uses these earphones to block out the distracting sounds of surrounding cubicle workers. I can attest to the fact that the headphones can passively attenuate environmental sound pretty nicely—on an airplane ride, I could distinctly hear movie dialog and spoken song lyrics, even in quieter parts.
One common thread on Amazon.com reviews for these headphones is that the included cord is both durable and tangle-free. I found this to be very true. The cord is flat, with a bit of matted texture added for good grippability. It is easy to wrap up the cord or simply stuff it into the included hard imitation-leather case.
The headphones themselves are built of a blend of plastic and some sort of aluminum (the part that says 'Sony'). There's a little bump on the left earphone, for help identifying it when inserting the phones into your ears. The 'L' and 'R' are printed on, and I imagine they could wear off at some point.
The phones stick out from my ear just a bit too much for my liking, but I'm happier with better sound quality as opposed to high-fashion headphones.
I finally decided to return the headphones, as they were a bit too bass-heavy for my liking and for my musical tastes. However, for the price, you can't beat these headphones if you can tolerate the bass. The headphones have an excellent build quality, and they can give new life to many songs.
Buy the Sony MDR-XB40EX Extra Bass Earbuds on Amazon.com for about $55.