I was at a beseiged yesterday. It was kind of cool to go outside as they were approaching and watch the swarm run by you (you could attack but the monsters would only stop for a few seconds then start running to town again.) Inside was as the others have said, many people, lots of lag, and not many monsters that came in waves. 4-5 would pop, everyone would kill them, then another 4-5, etc. I was able to engage and beat up on some although others in my ls never even saw them. BLM would just thundaga and the mobs would die quickly it seemed like.
I think the concept would be better if there were a lot more monsters and they didn't all spawn in the same general area... maybe have them coming over the walls, etc. Maybe it will get better as time goes on and less people are interested. Yesterday on our server 2 armies were marching at the same time, although it seemed they hit at different times.