Personal opinion: NOT COOL! Especially in the same lane when there's two lanes moving in the same direction.
Happened today on Hwy 91. I was going about 110km in the left lane and was passed by two speedsters going much faster on my left side (I'd guess about 140km+). As I was already passing most traffic myself, I was surprised by the first guy passing me on my left in the same lane.
These guys were putting themselves (and me) in a dangerous situation. What if there was a pothole or debris on the road that I needed to avoid? It would have been a nasty site if we bumped!
I don't mind being passed by someone in the right lane, but not the same lane. For those who like the speed - go to a race track as our city streets and highways aren't really designed for racing.