It's varied. Sometimes it feels like no lag, possibly because I have been playing P:M and Brawl on an HD tv so I just don't notice anymore, but sometimes the input lag is worse than brawl, and I had a wait like a whole 2 seconds for an input to go through.
However, my household has acquired much more wireless items since Brawls time, going from 2 computers to 3 computers, a bunch of smartphones, a wii u and another 3ds, and it's still better than Brawls online majority of the time. And the lag has never prevented me from doing something I originally intended to do anyway.
If it comes down to it i just end the game by suiciding and just leave the room if the lag's really that bad.