As a Catholic member of a Catholic "megachurch," I'm compelled to remind people that painting anything with a broad brush is likely to result in you being wrong about at least something.
My parish - a CATHOLIC parish - qualifes as a "megachurch." We have well in excess of 10,000 members and I read scripture to about 2800 people while serving as a lector during the 11:00 mass this past Sunday. Our theology is sound and we do have a honest priest who delivers Christ's message (we actually have two of them as well as two deacons).
Bigger doesn't always mean better, but it doesn't always mean "badder," either.
It probably also has a certain timeless quality about it that the showplace megachurches don't. My parish also qualifies as a megachurch, even though the parish is over 100 years old. The church itself is beautiful and feels timeless. It's awe-inspiring - what a church should be. True crystal cathedral type mega churches are far more ephemeral.
it's not a question of bigger is bad. It's just that many of these big groups that congregate in soccer stadiums etc. are no more than song and dance acts, like Benny Hinn