The problem with it is that it's not a very good design from a mechanical standpoint. It's hard to get the thing in your ear, and when you do you're not going to have good fit or isolation. The driver isn't the best, but it's not that bad either - but it's not used to it's maximum potential. Most of the mods revolve around a) changing the stock foamie to someting that isolates, like an earplug or a tri-flange tip from another canalphone and b) dampening the driver, either by padding the venting ports on the rear of the plug or padding the sound tube, in order to tame the shrill highs and boomy bass.
Fully modded, they're not that bad. But it'll take some work, so you might as well look into Sharp MD-33's or Shure e2c's instead.
I was merrily romping along with my Koss Plugs for at least a year before I happened upon this infernal website. Now, Shure E2s, E3s, E5s, Ety ER4Ps (P->S Cable) & Westone UM2s later (along with numerous DAPs and headphone amps)............I can truthfully say the Plugs suck. They just didn't cause the suction sound that seems to continuously eminate from my wallet.
from the reviews ive read everyone seems to agree that they suck, if you still dont mind then save yourself the money and buy the radioshack version (cheaper). if not id say increase you budget and buy the shure e2c.
a friend of mine has a set of plugs, and i got to try them. none of the foam tips it came with worked worth a damn, but shure E2c foams more or less fit on and aren't terrible with them. i think the phones are kinda crappy, but not bad for the $10 they sell for. i wouldn't buy a pair, though.
i also had the plugs and loved the sound i had after modding them before i came to this website. they sounded great modded after having the sony mdr-ex71s. however, after buying the e2cs and then the e3cs, they sound like such crap even with the mods. i would go with e2cs if you can find a deal on them. they're worth it.
if you do decide to go with them though, you might want to wait. ultra soft sleeves have just been developed and tested for them and are about to be released soon, so you could get them for free with the earphones, instead of paying extra later.