PC magazine put a demo out, multiplayer is definitely where it shines, but you can definitely get down in single player.
The sinking hours into it thing can be applied to pretty much any game, my mate's made £30 after people paid him to give them a shit ton of 'decent items', he just got a list of what they wanted, made up a price, got their details + saves, went into his modded .jar, used TooManyItems to get all the stuff they wanted in about 10 clicks, and then give them back their saves with a load of diamonds, gold, etc. Made about £30 for literally 10 minutes work, I'm fair tempted to do it myself but it's such little effort I almost feel bad.
I've got Terraria as well, but I'm getting bored with having gotten no decent items for ages and networking problems.