I guess all earphone's durability depends on how you use them. Personally I've been using my PT850 about 5-6 hours a day for almost 5 months, and they're still as good as new. Granted, I take good care of them, but I wouldn't have a right to criticize if I didn't do my part, right? :P
Regarding sound quality, I disagree with those who say it's bad. In fact, I was very surprised with the PT850 when heard them for the first time. Compared to other bundled earphones, they must be the best I've heard so far. Of course they are not even close to audiophile quality, but they should still be better than anything you can buy at under 25€.. they sure beat my Sony MDR-EX51LP Fontopia hands down, and those cost me 35€ a couple of years ago.
IMO, if you want to replace the PT850 for better audio quality, you can't do it with cheap phones... at that point maybe it makes send to go a step further, spend a little more, and go for the MylarOne X3i's or the Audio Technica ATH-CK7 (these are probably my next phones
).