AS400

How difficult is it to manage this system?

Depends....how much experience do you have with AIX? they're not bad, but you could get very complex with the VMs and stuff.

Once they're up and running they're rock solid.

like anything from IBM...

They are TANKS...

All the casinos use iSeries as they're now called. They are rock solid. Usually they are run by old school AS400 guys. I've done some basic admin on them and its pretty simple cli menu driven stuff.