I'm a huge fan of the Thief series, got all 3 of them. However, Thief 3 didn't run very well on my system as I had an ATI card, and it has a problem with ATI cards. The card was powerful enough to run it decently, but I read the readme and it did say that ATI cards can't run Thief 3 well for some reason. I stuck with it, though, it ran ok most of the time. I played quite a few of the missions, then I had a problem with my most recent save game. The newest working save game was about 4/5 long missions back. :x Gave up after that, buuuut, I'm getting a new GeForce 6600 graphics card tomorrow, so I'll be going back to Thief 3.
How is the Thief 3 editor, not used it yet. I did some editing with DromEd on Thief 1 and 2, though it's not the easiest program to use, I could at least get my head round it, which is more than can be said for most editors.