Rough interpretation of disc ratings:
0 discs: Only useful for practicing. Useless for an actual round.
.5 discs: Only playable if you're in the neighborhood, but there is nothing interesting or enjoyable about the course.
1 disc: A boring and/or beginner oriented 9-holer -- A course with several major flaws. I would only play here if I was nearby and desperate for a DG fix.
1.5 discs: A below average course with multiple flaws. Uninspiring, unchallenging, poor course conditions, or all of the above.
2 discs: Below average, but still worth playing. Would not go out of my way to play a round here, but there are aspects of the course that are enjoyable.
2.5 discs: An average 18-hole course (or slightly above average 9-hole course. Fun or challenging enough to play multiple times.
3 discs: A good course. Fun, challenging, and replayable. Maybe deficient in one or two areas, or too small to provide the type of challenge of a 4-star.
3.5 discs: The highest score I could see giving a 9-holer. Distance, shot variety, a signature hole, and well maintained. Either lacks character or missing one or two aspects of the higher rated categories.
4 discs: An excellent course. Fun, challenging, plenty of variety, and at least one signature hole. A must play if you're in the area.
4.5 discs: An elite course, likely top 1 or 2 in its respective state. A course worth a one day's drive to. Free of any major flaws, just lacking the unique character or mystique of a 5-star.
5 discs: An unforgettable, jaw-dropping course that has a character completely unique unto itself and is free of any major flaw. Immensely fun, challenging, and geographically diverse. The type of course worthy of planning a road trip or vacation to.