Granted, I am not streaming movies, or even music, but I have no problems playing any online games. Sure, electrically the Ethernet cable is superior, but for online gaming, my view is that it is not necessary in most environments (noisy environments may require a LAN cable, though).
That may be the case if it isn't set up properly... I have zero issues with either my PS3 or my PC over wireless. I run driving sims with my PC and I have actually done a head-to-head comparison and there was no delta with the wireless connection.