If you're not completely sold on the concept, head over to Minecraft.net and give Classic Mode a try. I really hate to point new players to Classic, as you can probably guess, because I don't find Classic fun at all, but it's currently the only way you can try any aspect of the gameplay for free (sadly, at the time of writing, Classic Mode is unavailable due to the whole server crash and all). Just make sure you know going into it that Survival is 999x better, so if it puts even the tiniest smile on your face when you build something neat, you should rip your wallet open and give Notch your money so you can play Survival. He's a super talented dude that did exactly what I was hoping someone would do with Infiniminer. He stepped up and took it to the next level. Shit, he took it to the next five levels. And he's still leveling.
Minecraft is currently �9.95 (which is about $13) and will jump to �20 once it hits beta, so buy it now! Once you've paid for an account, you're set for good, so there's no need to worry about paying more than $13 for a lifetime of Minecraft fun. Seriously, even if you get bored of what you're currently capable of (how could you?), there's always next week's update to look forward to. And one spectacular day, this will be a full, retail-quality game with achievements (don't judge me) and all kinds of different modes. I can't fucking wait.
Here's a fanmade trailer that's already been frontpaged, but I don't care. It's too awesome to miss.