I am possibly going to cause some shock and horror with the following comments.
I will point out I am an AFOL at heart although I do also build other brand construction toys and often enjoy them although not as much as LEGO.
I don't see Mega Bloks trying to compete with LEGO at all, Infact they are polar opposites in terms of themes and seem to be doing themes that LEGO has not and will not get involved with.
EG Mega Bloks Call Of Duty (Obviously LEGO is not interested in such a theme due to war violence and guns)
Mega Bloks Assasins Creed (Coming soon but again LEGO would not want to touch on such a theme due to the nature of it)
To me LEGO represents all thats good and well in the world with happy little towns mythical creatures and where the worst charachter you will meet is a robber, other than that it is all happy land and rainbows and unicorns.
Mega Bloks on the other hand seems to be going for a different demographic altoghether (older gamer type folks who probably have zero interest in LEGO but will buy a set because it is something from a video game)
I don't think you will ever find someone who says that LEGO is not the best quality construction set on the market.
However
Should you want some amazing detailed guns and weapons that look realistic then Mega Bloks has that for you.
Believe it or not it was a charachter building set that brought me out of the dark ages (a small Doctor Who Set), I did not buy it because I wanted a construction toy but instead bought it because it was Doctor Who, I sat and built the set and then the memories of how I loved LEGO came flooding back, I think any brand set that brings back such memories of LEGO is a good thing for LEGO and good for expanding the number of AFOL in the world.
Just my 5c but feel free to throw the full 5c back at me :-)