KVM-Over-IP

I'm looking at improving our current data-centre systems and moving away from remote desktop control.

Has anyone any experience (positive or negative!) of Avocent equipment? How does it compare to someone like Raritan, what holes do I need to poke in my firewalls and rotuer ACLS? Is it easy to use, what are the bandwidth requirements like?
I know they offer a live demo which I have lined up for next week, but I would rather be going in to this fore-armed!

It has a high capital cost (GBP 18K) but depending on the quality and the level of service that it can provide, it can basically save me a 150 mile round trip if a server needs reinstalled and gives us a central management console.