Whilst I am somewhat sympathetic to the cause (Megaupload held some work that I helped to create, massive community-oriented and totally original work - tactical guides for Warhammer and suchlike, as well as being a great hub for game mods) I still think this is very stupid.
I have always disliked Anonymous - I think they are more trouble than they're worth and keep doing things just because they can, rather than because it is right for the situation - but this time I think they have gone way too far. All I can see this doing is proving to people (both politicians and the public) that government-mandated policies such as SOPA, PIPA, ACTA and so on are absolutely essential, as it would stop such acts. This is what people will remember when criticisms of SOPA/PIPA are raised, and it will stereotype all those who are against the bills as hackers and pirates solely looking to protect their illegal fun/livelihoods. It may even encourage foreign governments (to the USA) to make similar bills.
In short, congratulations Anonymous, you have just done more to make the case for tighter internet regulation/censorship than any politician or lobby group could possibly have ever done.