It reminds me of Virtual Springfield in that the joy of the game comes less from the gameplay mechanics, but more from poking at the game to get a reaction. The scenario and writing elevate the mundane, with the mundane being the fetch quests and side quests that in any other game would be busywork, but in this it's a chance to see a cameo or two or just some funny dialogue. It's a little light mechanically, reminding me somewhat of the Costume Quest games.
I've also just realised halfway through day 2 that I haven't used any of my perks. A notification popped up after 45 friends, at which point I thought "shit, perks, completely forgot about them." I'd be tempted to up the difficulty, but honestly, a challenge isn't something I particularly want out of this game. Still, it's exceeded my expectations. Brilliant job, Obsidian. Only one minor bug so far in eight hour plays as well.
I just love how confident Stick of Truth is in being both a video game and faithful to the series. The issue I have with borderline every movie/TV franchise game ever is that they come across detached and phoned in, like the game doesn't know how to translate the filmed material and style to a video game environment. Stick of Truth on the other hand doesn't seem overly reliant on gimmicks, repeated jokes, or homages to be "South Park". It has these, but like others have said also stands as an episode/season in itself. It's like a season has been written in interactive form instead of linear form as it normally would, and enough attention paid to that interactive part to make it enjoyable. The two components come together wonderfully.
It's very clever in how it moves pretty much all the fan service into the background of the game, whilst maintaining a core of original writing not at all reliant on mining the series huge archive of episodes.