I find myself only going after "Best of"/"Compilation" discs as it pertains to Pop/Rock artists. I have a very eclectic taste, but it doesn't reach as far with the aforementioned. That's probably due to radio dictating what is, and what is not considered popular. For instance, I just went back and listened to some obscure Bryan Adams/Rick Springfield stuff over the weekend. There was some stuff I liked, that I hadn't known about, but I tend to be a little more picky.
When it comes to metal, I think I tend to give bands a lot more leeway when it comes to the broader appeal of an album. I think this has a lot to do with putting a deemphasis on what would be considered a popular metal song or hit. I've always tended to like stuff from metal bands that isn't the flavor of the month. So, when it comes to metal or anything non radio friendly, I go after the albums for those bands. I might buy a "Greatest hits" album if it has been re-mastered, and their old catalog still hasn't been updated.