RDR is different than GTA IV, but at the end of the day it's still a Rockstar game, so you're getting an overhyped, "open-world", third person shooter.
That said...$20 is a good price for that. I paid $60 when it came out after falling for the Rockstar hype machine, and I'll never make that mistake again.
Platinum Wish List: Star Ocean: TLH; Disgaea 3; Disgaea 4
My New Year's resolution will be to not spend more than $30 on a game in 2013.