Had a go with the terrain editor, seems a similar method to UT2004 but not very straightforward or intuitive (but I've not mapped since 2001 so it probably won't be). But the scripting... ugh. I took a look at the player script, and the things that struck me first were the lack of a clear inheritance system (probably just need to look harder - at least that's what I hope), and some really badly thought out things such as having the ammo count in the player itself, not in the ammo or inventory. If that's an indication of what the rest of the scripts are like, I really wouldn't consider developing anything for it. Also, I can't find any way of exporting all scripts, and doing them manually is more than tedious.
This was all before I went out and bought Doom 3, installed it, ran it once got a BSOD, couldn't boot into windows, reflashed my video card ROM, almost reinstalled everything before switching the power off and on to miraculously fix it, and played it for a bit. But that's another story.