I've been playing since Alpha, though I burned out and took a long break. I started a new world a couple of days after the official launch in November and I'm still working on that world now. I'm pretty much only a single survival player - hard mode always.
(I have tried Hardcore a few times and enjoyed it, but I put way too much work into my builds to risk losing them all the time to a single random creeper...)
I've been building pretty much entirely underground, building minecart tunnels in each direction. So far north, south and east are sitting at around 1km each, though west is rather slower going, since upper west runs through an abandoned mineshaft - and I hate clearing those things out unless I'm in the mood for mayhem - and lower west runs through a ridiculously huge cave that is impossible to light properly and spawns huge numbers of monsters.