Man this game is addictive. Even though the design sticks pretty close to the GTA mold, there's so much side stuff that keeps the game feeling fresh (and unlike GTA, most of the side content is both fun and optional). The treasure maps are one of the cooler ideas I've seen in an open-world game, and that first one was extremely satisfying to find.
It feels like the most polished open-world game I've played, too. The draw distance is great, the pop-in has been minimal so far, and it doesn't seem to have a lot of the "open-world jank" like cars/people spawning in front of you or vanishing when you turn your back. And of course the animation and voice acting are top-notch Rockstar quality, and Euphoria is present to full effect.
All in all I'm really loving what I've played so far... and I haven't even really touched the multiplayer yet.