Just thought I'd post a few pictures of my SP minecrafting...

I made a lovely Tower of Doom, and am now trying to quarry down to bedrock so I can have a bottomless (sort-of) pit.

Survival Multiplayer is currently pretty buggy, you cannot receive any damage, railroads do not work, redstone circuits bugs out often, iron doors cannot open within 10 blocks of a torch, water does not work like in Singleplayer but dissapears after a short while if placed with a bucket (which may be to avoid griefing), all in all lots of stuff to be fixed, but it should be part of the halloween update so it's all good, even with the bugs it's still great fun to build something with your friends! Your server is a Survival server though, it's just the bugs in it that make you think it's creative, in creative you can just place blocks at will everywhere, in survival it plays like singleplayer Alpha, you can enable mobs by editing the server.properties file in your server folder, but you can still not take any damage, so it'll be an annoyance.

This game (or whatever it is) really looks interesting, but i have a few questions to which i can't find clear answers on the page:

Do i need to be online to play? Somewhere it says something about being DRM-free but somewhere else it says i need to authenticate with their servers at least once per installation.

Is it easy to update the game? It says there are regular updates, so is there some kind of automatic updater or do i have to manually patch every time a new version is released?

Can I play this game in single player mode without having to set up a private server or so? I always read something about servers to play on.

How stable is this Alpha version? Can i build up something without having to worry about constant crashes?

Is there some kind of sandbox mode where you don't need to mine and craft stuff but have it all available from the start?

Thanks for your answers! :)

There's authentication in the same sense as any other download service, in that you have to log in to get the thing in the first place. You get a launcher, and the first time it's run, you have to log in so it can download the game files. After that, it can be played offline. I'm pretty sure the game files can be copied between machines without problems.
Yes, the launcher also handles updates. You'll have to be logged in for that as well.
Servers are just for multi player. For single player, you just pick a save slot to create a world in, and go.
Single player is very stable. I've never seen a crash. I hear multi player can be pretty unreliable, though.
There's creative mode, but it isn't supported any more, and there isn't a downloadable version. It's free though.
Edit: survival mode can be set to "peaceful" which removes the monsters and lets you concentrate on building. You'll still have to mine the stuff you need, but you won't have to worry about getting jumped by creepers.

For those averse to putting Java on their systems because of security concerns, note that you can also play Minecraft without having Java installed by running the "Linux / Other" download with a combination of Java Portable and Java Portable Launcher. This way Java is only active when you're actually playing the game (and gone as soon as you close it), essentially negating any potential security threats. You also don't get the browser plugin and annoying background updater that come with a normal Java install. A nice bonus is that you can chuck the whole folder onto a flash drive or whatever to play it anywhere you like. :)

Indeed it can--if you copy all the Minecraft data (rather than just the saves) you don't even need to authenticate again.

Why the Linux one? Can't Java Portable work with the PC version?

This made me laugh. ;)
You're one of a kind, Michael!:D

Well you see, I'm a noob as you can tell. ON EVERYTHING.

Now excuse me while I figure out how you open bottle caps.

Just to make it clear, i don't know why You need the Linux version.
Perhaps, Java Portable works a bit different, and doesn't like the standard Windows java?
That's why i laughed, see, because You wrote "PC" instead of "Windows". As if Linux was not made for PCs too.;)

Good luck with the bottle. ;P

Java Portable is the real Windows Java, but the problem is that you can't execute EXEs directly in Java, only JARs. The Windows download is simply a JAR in a fancy native Windows wrapper, so the game itself is identical regardless of which method you use to run it.

I don't know if this was linked before, if it was, I'm sorry
http://www.minecraftforum.net/viewtopic.php?f=3&t=49256

Ahh right thanks, that's why the pool didn't work!

I would like to play on a server vs mobs.. yes...
I want some TNT.

Well, actually, gunpowder randomly appears as dungeon loot (I left what I found in the main tower's entrance storage, I think), but as you can imagine this makes it insanely rare (especially since you need five parts for a single TNT block).

Building cool stuff while also fending off enemies would be great. :) Hopefully Notch is doing some work on SMP for the Halloween update.

Thanks for the answers! Think i'm gonna buy it soon... :)