I've gone with a grade school system, as it resonates with me personally, and doesn't result in the "7,8,9" problem that 0-10 scales have. I'll use +'s and -'s occasionally for a bit more precision. Here's my breakdown of what my scores mean to me:

A+: amazing, a must play
A: a great game, worth your cash
B: a solid game, try it
C: average, skip it probably
D: not good, avoid
F: horrible, get angry