I finished GTA the other day, and now I'm shooting toward 100% -- Don't know how many more of the offline achievements I want to go for though ... I know I'll never get 70 golds, and even then, I'm not sure I want to go through the rigorous task of obtaining the letter pieces or the spaceship parts, but man...when you're that far into it, you might as well right? I only picked up one letter piece in the story mode because Trevor woke up drunk next to someone's swimming pool in a loading screen, and there happened to be a letter piece glowing behind his head.
I enjoyed it, but thought it was weird that the story mode took so long to develop, and basically closed up so quickly. I thought they'd drag out Option C a bit more to be honest with you.
The hobbies and pastimes are the one thing holding me back right now (I'd only completed 3 at the end of story mode.) Flight school was a pain in the ***. I hate piloting helicopters in GTA V...the planes are ok once you get used to the controls, but man, the helicopters are a **** show compared to GTA IV.