I couldn't seem to find a way to play this in full screen, the online version is pretty lame. Should I just spend some Sacajaweas and buy it?
 
EDIT: Crap I go to impacient and while everone was replying I was playing minecraft. lol I bought it like 5 min after making this thread. TIMEWASTERCRAFT!

No demo since you have to have a paid account to play the game.  It is difficult to tell everybody to buy it.  The game is still in alpha and is at the discounted price which will increase one multiplayer works and the game moves to beta so it would probably be best to buy it now. 
 
That being said, the game is only as good as you make it. You need to enjoy using the tools in the game to build or mine and to be honest, the game does begin to slow down once you lose interest in building and mining. I can only hope that with a working multiplayer along with new items introduced, I'll start enjoying the game more. 
 
Watch Youtube videos and decide from there is all I can recommend. If you are slightly interested, I say, go for it.

Yup, can't login. His server has been getting destroyed from all the new users. I would say take advantage of it now and try playing Offline.

" @Jack_Daniels:  It lets me play without paying for some reason. It says my registration is invalid, then just asks if I want to play offline. Try creating an account, or maybe just download it and type in a bunch of jibberish. It should still let you play offline... which is the best way to play right now. If you like it at all, I'd recommend buying it because you will therefore be purchasing all future versions for the low current price. Plus, there's no DRM. "

Just buy it, although his server has been a little fucked up lately so He might not actually be able to give you the game for a few days.

You can download and play the alpha without purchasing it first, although you won't have access to some of the features.