I just recently discovered the PvE server via a friend. I've been building a massive underground complex near Sealab 2020, I'm on there almost every day.
I tend to stick close to Endor, mostly because I don't know my way around and I don't want to get lost. I've been grinding in the nether lately. My MC name is the same as my screen name here.
It does seem a bit empty most of the time, but I'm cool with that. It's like I'm playing a massive single player game, lol!
I know the server's getting wiped soon so that we can all start over. Until that happens, I agree that structures built by players that haven't been online in months should be re-purposed. Even if it was as simple as removing the locks from their chests >: )