I use 1-5 stars scale with half-stars (though initially I didn't want to use halfs, but eventually they have appeared to be useful).
Only albums are rated as a whole.
Weak ratings appear, because I tend to have complete discographies of bands, especially ones I like.
1 - Mistake. Bad music.
2 - Sounds like OK, just I don't like it.
2.5 - Great album they say, classic, but still I don't like it (rare rating, artificial - not based on my personal taste).
3 - Definitely worth listening again.
3.5 - Halfway between 3 and 4
4 - Great record in every aspect
4.5 - Halfway between 4 and 5
5 - Epitome of a genre (like 4 with a strong personal preference, not related to any common perception of an album)
Now, if anyone asks me for good albums to listen, I select 2.5, 4, 4.5, 5
If I want some undistracting muzak, I select 3, 3.5
If I want only personal favourites, I select 5
If I want to have a good listen, but not music I know by heart, I select 4, 4.5 or 3.5, 4, 4.5
etc etc
1-2.5 stay in ghetto of badness forever.
3 and 5 are very unlikely to change after first rating
3.5, 4, 4.5 are subject to change at subsequent listens in any direction.