It's a massive time sink, be warned!
I host a server at work for me and a couple of friends, I played some a Saturday, went back last to see what had happened, one guy had built a massive tower. Looks awesome.
I must have been on ages because I'd dug miles and miles of tunnels, took me ages to navigate them all, there's three massive 'natural' caverns it runs into along the way that are full of zombies and skeletons - same you can't kill them or get hurt =/ Found about 30 diamonds as well - no wonder my weekend seemed quick!
I've joined up the two main entrances with a sky-bridge, which pretty much goes from one side of our island to the other.