I completely agree, mucking around in that engine was extremely instructive. I actually bought the thick paperback book for it and starting buying PC Gamer with the demo discs because they had extra levels people made for awhile, with one outstanding map of a 4-city block with trucks that drove around without crashing into each other, and you could ride them and gun down others, extremely fun.
It was very interesting to see how much of the game could be edited from a plain text file, and what really blew my mind what experimenting quickly with non-euclidian spaces: you could make a square room, put an "archway" portal in the center, and walking through the east side would do nothing/see nothing, but build a room off the portal where you see a space that couldn't possibly exist walking through the west side.