I've had the MX 500's for about 6 weeks, so I can't really say too much about reliability, but so far so good, including the volume slider, though honestly it feels a little cheap.
As far as the sound goes, I'm very impressed with them. After putting the foamies on, that is. I'd read a review of some other earbud here where the guy said the foamies just made them feel bigger (too big) and sound muddy, so at first I put them in naked (the earbuds...). I was EXTREMELY dissapointed. They were, to me, the worst sounding 'phones of any kind I'd heard. They had virtually no bass at all and sounded basically like this: imagine an aluminum can vibrating along to a very loud concert nearby - now subtract the concert, but keep the sound coming from the can. And, they felt big even without the foamies. I was so shocked by this letdown that I didn't try them with foamies for two days - that just sat there, pathetically. Then, on the third day, they rose again from the dead (sorry). On a whim, I put the foamies on and stuck'em in. Wow. All of the sudden there was bass. The foamies really help couple them to your ears, I guess. And, they were much more comfortable because I was no longer trying to push them in deep to get bass. I was very happy to feel so stupid about the whole thing. So, now I believe them to be the best $20 I've spent since my first date with my girlfriend (wow, I'm a cheap person!
). The best thing about them is their balance, I think. My 'better' headphones (Sony MDR-CD77's - oldies, but goodies) are sometimes displaced in my listening time because, while I can get better sound from them, it takes a fair amount of tweaking with frequencey levels and such before I'm paying attention to the music instead of the headphones. With the 500's, I just set everything to 'flat' and plug in. Someone else said they didn't have much bass, but I think they have plenty - it just doesn't go real deep, but you really can't expect great bass in this form factor/price range (I have Audio Technica aluminum sticks on the way, so we'll see if you can pay your way to excellent sound in earbuds). So, all in all, I highly recommend and enjoy them.