I have only ever purchased one Megablock set and that was the Viking boat quite a few years ago. I ended up tossing out the ship after about two weeks. That said, I have come across quite a bit of Megablock product through garage sales and Internet auctions. I have also been observing Megablock set design for years. That said, here is my two cents:
Megablocks have several reasons for being inferior.
1) They are not Lego
2) They were originally created as a "rip" off brand of Lego
3) The plastic used to make Megablocks is more brittle and does not hold up as well over time.
4) Manufacturing error allowances are greater for Megablocks than Lego; this means the bricks do not always stay together when built.
5) Megablocks set design sways very strongly towards juniorization.
Definitely glaring reasons to avoid Megablocks like the plague. That being said, Megablocks as a brand has recently made some vast improvements in set design, at a time when it seems Lego has forgotten our favorite little theme.
The Dragon Line or whatever its name is; towers above all the Lego castle designs we have seen in the last 6 years or so. Minifig packs, exciting packaging and fantastical elements have paved the way for throngs of castle fans to renew their interest in the hobby sans Lego. I personally know several people who have stopped buying Lego altogether in order to focus on building Megablock armies.
This is appalling, I know, and it is indicative of how Lego has consistently dropped the ball in recent years. Even I have been tempted away from my favored brick by the fantastic potential in these new castle themed sets. Now I haven't gone as far as some, I have yet to purchase any, but the temptation is there.
I can say without question that if Lego takes the castle theme down the rumored Bionical-like path, I will jump ship. I will stop buying Lego at all and I will start purchasing the Megablock dragon line.
I have decided that the only way to affect change in the way Lego treats its castle fans will be to pump up the sales figures for the Megablocks dragon line.
Now will I keep any blocks etc... from the dragon line? No, that won't change, but I will keep the dragons, minifigs and weapons from these sets. I will soil my collection with off-brand product. For the first time in 10 years, I will buy building sets without the little red Lego logo. Pity the rest of my collection that will have to adjust to the influx of off-brand figures. Pity the Lego dragons that will be decimated by the bigger flashier Megablocks dragons. Most of all follow me into oblivion and join me in protest if this Bionical/Castle rumor is true.
Gee my two cents is long winded.
After a long absence, I have returned. I can't wait to start building again.