You know what I'm talking about: a game gets a mediocre-to-bad review, you end up playing it, and find to your utter astonishment... that you can't put it down.

For me, it would be the Sonic Adventure series on the GC (from the DC). They got average to poor reviews and I played each of them for about 35-40 hours each. If I picked them up again, I would probably find them just as fun (though not without their flaws).

I honestly cannot think of anything. These days, I'm so fickle with games that almost everything that I play tends to score in at least the 70’s margin.

Don’t get me wrong though, I don’t go around looking only for high scoring games to play or anything like that.

I don't really remember a lot of the ratings, but some games that I don't remember having a lot of hype that I really enjoyed:

Hudson's Adventure Island - NES
Jurassic Park - SNES
Illusion of Gaia - SNES
Shadows Over Riva - PC
Lands of Lore II - PC
Magic the Gathering: Duel of the Plainswalkers - PC
Star Wars: Shadows of the Empire - N64 (pretty mediocre reviews if I remember correctly)
Mark of Kri - PS2
Rise of the Kasai - PS2
Suikoden IV - PS2

Off the top of my head.

Most games I own get pretty good ratings except one of my favorite games: Legend of The Dragoon. From what I remember it got pretty mediocre reviews but I could not stop playing that damn game.