I haven't done it much, but if you find a decent server in Flightsim with someone working the tower, it can be a great educational
aid. Some of the servers are passworded, which would imply that some folks are pursuing some quality control.
I can see this being a great boon for student pilots, wannabes, and even experienced folks who'd like to brush up.
The best one I've seen so far (open, that is), was set at Wittman, and a young lady was working the tower. Aside from the occasional
jerk who'd log in and bounce around the active runway (with his crash effects off... so he didn't disappear when he hit the ground),
it worked really well.
You can also share aircraft, and I was invited into another guy's plane to show him how to land without cracking up. I felt pretty honored,
though it was odd since he had all his settings set to 'easy'.
The gal in the tower wouldn't hesitate to let us know when we missed a call, so I figured a controlled server with some serious folks
who really wanted to do well would be a very very good thing.
I haven't tried the ATC function yet, but I will soon, and I'll let ya know what I think (not that I've ever worked a tower...)