I go to my local wal mart about twice a week for legos. I've overheard the adult/parent conversation about megablocks and legos. The dad, a friend of mine from high school, not really a friend but a dude I knew, was trying to convince his son that the mega blocks halo stuff was simply nonsense and he would help said son with modding some parts so he could make his own little halo thing. I looked over and nodded at said dad. He looked at me and said to his son "See, even Jeff knows the difference, if you dont want to take it from me, take it from him." I have a bit of a reputation around town as the guy that held build events in his garage and at the local comic shop, the kid knew me from pokemon saturdays and agreed. I've also helped out older folks, grandparents get something for their grand child. Ran into one of them at one point after they bought their kid the space police hq and galactic enforce..big megablocks space ship. ((Lucky little kid)) They thanked me for the advice and said that he enjoyed both sets.
Speak up, explain what quality over quantity means. Parents will see the bigger bang for the buck all the time, but when it comes to a toy that'll last, lego is hand's down the best.