I got these headphones and also the MDR-EX51LP, I liked these better than the MDR-EX51LP because the sounds it produced was crisper and overall just much better.My only problem with these is that the chord is a little short, just a little over 3ft.
I didn't choose the MDR-EX51LP because that the bass was way too loud.I know that people like bass, but whenever you listen to songs with lyrics all you can hear is the bass, and the spoken stuff gets drowned a bit.
I like the MDR-ED21LP because it was just right, and it was actually louder.I use them on my IAudio CW200 and I usually set my volume to 25.When I have it in that volume level MDR-EX51LP is just too low to even hear it.Even though it goes into your ear canal it doesn't perform as loud as the the MDR-ED21LP.You can also hear some ruffling noise whenever the chord rubs in yourclothes, like the stetoscope effect, as another user reviewed.
For the MDR-ED21LP, everything is great, crisp, loud sound, great bass.My only complaint is the chord length.
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Sony MDR-ED21LP Fontopia In-The-Ear HeadphonesProduct Description: Super-light in-the-ear headphones Soft rubber earpiece for comfort Unique bass boster delivers wider frequency sound Anti-bacterial earpiece material Sonys Acoustic Twin Turbo Circuit for enhanced sound quality 16 mm drive units High-power neodymium magnet Fontopia(R) in-the-ear design : fits snugly, yet comfortably inside the ear; eliminates the headband so you can enjoy music while exercising, skating or dancing Soft rubber earpieces : provide greater stability so they're far less likely to shift or fallout, greater wearing comfort thanks to the broader distribution of contact Groove design/Bass Booster earpiece shape : with better fit means less sound is lost; better bass response at the same sound pressure levels as conventional designs; perfect for Sony Walkman(R) players with Groove bass control Anti-bacterial earpiece material : for worry-free wearing 16 mm Diameter Driver : delivers deeper bass, extended treble and higher fidelity, yet remains small enough to fit inside your ear PET diaphragms : ensure high rigidity for minimum distortion; low mass for extended high-frequency response out to 23,000 Hz Sony Acoustic Twin Turbo(R) Circuitry : uses sound chamber and ducts for improved efficiency, gets high sonic output from your portable stereo Neodymium magnet : for maximum energy in minimum size; ounce for ounce, Neodymium is far more powerful than conventional Samarium Cobalt or Aluminum magnets Supplied carrying pouch : neatly stores the ear-buds and headphone cord when not in use; ideal for carrying in a pocket or purse SUPPLIED ACCESSORIES Carrying pouch WEIGHT 0.18 oz. (5g), without cord
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I found that these fit equally well on both my Plantronics H101 headset and my Philips SBC HS500 headphones. 
