I've been to two Comicons and have had a great time at both. Whether it's sitting in and listening to different panels, browsing through the goods for sale, admiring the costumes, or hearing the guest stars speak there's always something to do while you're there. The last Comicon I went to had guest stars such as John Barrowman, Stephen Amell, Bruce Campbell, and Nathan Fillion. Sadly, we weren't able to see Nathan Fillion because they were at capacity in the room he was at when we tried to enter.
The main issue I have is that there are so many people attending the event that it can be really hard to even walk around, and if you don't come super early to the guest speaker shows you might not get in.