The downtime is to weed out players who get frustrated so that the server doesnt lag on Tuesday. So they dont have to buy new servers.
There was no problems at all Friday at 10am PST when the servers went live for the headstart but tonight when I try to play during prime time of course all hell has broken loose. I guess this could at least mean the game is popular so that's a good thing but I am a bit surprised the servers have crashed so badly.
Headstarts, I think, is something, Cryptic started with CoH (at least publicly). It was a chance to test the production systems with people playing as they would in live. Many MMORPG had "headstarts" but they were limited to hand picked people, usually very active beta players and friends of the developers. Cryptic was the first I ever saw using it as a sales tool to encourage people to pre-order.