It has that magic combination of art, humor, gameplay, and actual theme. It's 150+ hours for me, climbing toward all the Steam achievements (just got the Mamas Boy the other night).
Since you've already hit it pretty hard, I would add the DLC, it adds lots of Fun to the game. It does make performance take a major hit though, because of the way the developers used Flash.
My other piece of advice is: don't get spoiled. Don't read about the enemies, levels, powerups, etc. online. Just play the game and let it teach you.
I keep the wiki handy for a list of what all the powerups do (since I can never remember), but I can definitely see the fun in just tinkering with them if you're an "explorer" type.
Since you've already hit it pretty hard, I would add the DLC, it adds lots of Fun to the game. It does make performance take a major hit though, because of the way the developers used Flash.
My other piece of advice is: don't get spoiled. Don't read about the enemies, levels, powerups, etc. online. Just play the game and let it teach you.