Done with my 5-day Minecraft demo. Spent of bit of time figuring things out (then looking things up), tried to build a house, had a Creeper sneak up on me before I even saw it, got chased by amphibious spiders, died a couple of times. The usual, I'm sure.
If I do end up purchasing the full game, I'm presently undecided on how I want to play it. Leaving mobs turned on seems like the proper way to do it and would probably be interesting, but it's also rather annoying to wait for night to pass, and just building stuff in creative mode sounds fun too.