I'm trying out the Lenny release of Debian today. It works - not exactly out of the box; wireless, sound, and video take some tweaking.
It runs fast and I can see why it would be appealing to someone who likes to work on, or play with computers to get them to work with a Linux distribution. It's like playing a game or doing a crossword puzzle - something to keep your mind active.
I've tried Knoppix, DSL, Ubuntu and Debian so far. They all have their good points. I think it's all a matter of what you like and what you intend to do with a computer. For me Ubuntu is a nice compromise between usability and customization. I was curious to see what Debian was like since Ubuntu, Knoppix, DSL and a variety of other Linux distributions are based on Debian.
I'd say Debian would probably not be a good choice for someone who doesn't have any computer hacking skills or at least a lot of patience. I'll probably be sticking with good old Ubuntu but Lenny will be around anytime I want to check out what's going on with the source of a lot of the goodness in Linux distributions.
I think I started this about 20 hours ago so I better be going to bed now...