You should. It's cheap as hell, especially considering how long it lasts. 134 hours with the Wrath of the Lamb DLC, 75 hours without. If you do end up picking it up you might want to wait on WotL though because it makes even the early levels noticeably harder.
Blockland
fallout 3for a single player game, i put in over 700 hours. i've never come close with anything else.
Bastion. Binding of Isaac is a good runner-up.