Just took a three-day weekend to go to Orycon in Portland. It was my first large convention, and the first one where there was costuming. I couldn't resist that challenge, and brought down three steampunk outfits that went over extremely well. All of them featured Edwardian jewelry that had belonged to my grandmother.
The costumes at Orycon fell into four categories: Renaissance/Regency, space opera/superhero, fantasy/furry, and steampunk/pirate. With some interesting crossovers.
While it was fun to go parading around in costumes and getting photographed, I discovered that doing a con in costume has a couple of drawbacks. One is that you have to schlep a bunch of costumes, including boots and hats, to and from the hotel. The other is that you spend quite a bit of time talking with folks about costumes, and that means you have less time to talk with other people about reading, writing, science, politics, etc.
In the future, I'll probably alternate between going to cons as a reader/writer, as a concom volunteer, and as a costumer.