I went to Comicon way back in 2001, and I've been to 2 Blizzcons. Basically, there's going to be a lot of waiting in line... and there's going to be a lot of things to buy. But there are also interesting panels to attend and that's probably the key.
I'd look at the schedule and see if you can schedule out a list of panels to attend that you'd be interested in. If you can, go. If not, maybe it's not for you. Also, I advise returning to your hotel room (assuming you are staying in one during the con) and ordering food there rather than eating at the con. Prices there are going to be inflated.
Bear in mind that cons also have a notoriously bad rep for people showing up with little knowledge of or care about personal hygeine... so if that bothers you, you'll want to be aware of that.