We did it once and a lot of people seemed to be doing it (not to mean that it was too many and we had to fight for a beach chair, but enough to where we didn't feel like we were doing anything abnormal. There was even a miniature version of the Great Escape that happens on Main St each night. After the fireworks, a small but noticeable crowd made their way through the GCH and up to the monorail.)
We took the monorail, got there a little bit early, found an open beach chair, repositioned it a bit, and had a wonderfully romantic time. Just a bit further back were some hammocks that looked promising too. The one thing I would change though would be bringing a blanket. It was in April and while stifling hot during the day, the constant breeze coming off the water made it pretty brisk. Still a great time though and would highly recommend it!