Agh... typical. Yeah, you'll always get people like that. What you should do is swap armies for a game, you use that guy's nids and he uses your dark eldar, then you completely trash him with those nids, and see if he complains.
Basically, no, dark eldar are not an overpowered army. We've got our units that rock (warriors, wyches), we've got our good units (ravagers, incubi, reavers, warp beasts, raiders), we've got our sort of alright units (mandrakes, grotesques) and we've got our crud units (scourges, hellions), like all other 3rd edition armies.
The raider rush was crippled after 3rd edition, and half our codex is really not up to scratch game-wise so I don't think anyone reall has a leg to stand on if they try to claim dark eldar are overpowered, chaos, eldar and space marines are far more powerful armies than dark eldar, after those 3 armies, there is tau, necrons nids, guard and dark eldar on an even playing field... then the inquisition, then orks, except for spead freaks which are up with dark eldar and stuff. Speak freaks are the most competetive orks in games. So yeah, basically, I'd say dark eldar are equally powerful when compared to both nids and speak freaks. If you swap armies and get smacked, then it shows that you should change your list, or they should change theirs, if you win easily, they'll shut up. A lot of people just refuse to update their tactics when playing against dark eldar, sticking with their same old methods that work against other armies even when it's clear that they don't work... why can't they just adapt insted of whining? I can tell you that I will never play against nids the same way that I play against daemonhunters, so why should they treat dark eldar the same as space marines? It makes no sense.