I first went last year around the same time and had such a good time that I decided to come back every time the event is held. Not only does Comik Market attract around half a million people a year, it's also the main center for buying and exhibiting doujinshi (short manga mainly by independent artist groups that are called 'circles'). Especially for cosplayers this is the main opportunity to get noticed by the community and get some nice pictures taken.
In my experience the first day of Comiket is the least busiest and therefor the one that is most worth it to get pictures taken. On the first day I worked for IOEA, cosplaying as Ranko Kanzaki from the Cinderella Girls series. Because I was really busy throughout the day, I didn't get much time to hang out in the outdoors cosplay area(s).