The problem with alot of overated supposedly A grade games is the hype.
Developers release too much information or beta demos and the fun is over before it starts.
If a game can keep you playing for around 2 weeks of solid after work/school gameplay, then in my book it is a good game.
It has not only driven you to play the game, it has kept you interested and determined to complete the adventure.
Admittedly there are games out there that are designed to be a casual experience, such as singstar, or sports games, but they have a different entertainment value.
Hell half-life 2, took me 24 hours to complete, but no I dont think its overrated, Ive gone back through the game several times, as I enjoyed it so much.
I think you can tell most games stink within the first hour of gameplay....Although theres the exception of games that are rushed to be completed and the story cut short....ala Turok Evolution (a game that was meant to save the series, but failed misserably)
While Im thinking about it...Mafia for PC was a fracking amazing game