Is Garry's Mod worth buying?

There's a system that me and my buddies use when attempting to buy new games. The money per hour rule. $1 = 1 hour. If the game costs $80 I ask myself, "Will I play this game for 80 hours?". When it comes down to gmod, I have about 8000 hours and it's a $10 game... Even if you end up not liking it somehow it's a $10 game and I highly doubt you won't play it for at least 10 hours.