Pros
I interned at WebCongress last summer and it was a great learning experience. We were working on an event for Peru but we were working from Miami. It made most of our work confined to the computer, communicating with potential partners and sponsors via Skype. Because its a startup, its run like a small business in that you can have direct access to management for direction, advise, and even giving them suggestions. Casual work attire is also a plus. Overall a very laid back environment but it never took away from our professionalism.
Cons
Behind a desk ALL day! Some people can do it with no problem but it just wasn't for me. If you don't live in downtown, the drive is terrible.