This month's RPG Blog Carnival, hosted by Dice Monkey, is on established settings, in contrast to home-grown settings. Most of the time, I've used the latter. I've only used established settings twice. OK, four times. Four and a half.
Let's get the half a setting out of the way: very briefly, I ran a GURPS game on a BBS using a setting that appeared in Pyramid (I think. Or maybe it was in Space Gamer?) It was called The Hole. I call it half a setting because it was a very small locale with not a lot of background, more of a setting concept. Plus, we didn't get very far; it was the problems with running this play-by-BBS game (and playing in another one) that turned me off online gaming. But I digress. Because of the shortness of the game and the fact that almost none of the established setting details ever made it into play, I can't really comment on the quality of that setting.
The two settings I wasn't going to count at first were actually tightly-linked game-and-setting combos: Call of Cthulhu and Stormbringer. These games were pretty brief, too, but not as brief as the BBS game. Plus, I actually used the Cthulhu Mythos twice outside of a Chaosium game, so that qualifies it as a full setting.
One of the two remaining settings was the Yrth setting for GURPS, including the tie-in to GURPS Infinite Worlds. It's a fairly good setting, since it assumes actual medieval Earthlings transplanted with their cultures to a world with elves, dwarves and orcs. There's a map, and some sketch descriptions of the political break-down. The other was the Cidri setting for The Fantasy Trip, but the details of the setting never came into play, so maybe that doesn't count.
Still, there's something I learned from using these various settings. The purpose of an established setting is not really just to reduce the GM's workload, but mainly to provide a context for the players. They need to know what backgrounds are possible, what their characters would already know, what kind of things to expect. Fairly popular fictional settings, like Stormbringer, adapted to RPG form are pretty good for this, although not everyone has read or seen every fictional work (no matter how vehement some of our number demand that this not be so.) The Cthulhu Mythos may be overused, but it's one of the best established settings, regardless of your opinion of that setting. It's something practically everyone knows.
The other approach is to base cultural details on well-known analogues (historical or popular fiction) and provide summaries for the players. Yrth does this. The problem is that, as I learned, not every player will buy into every element of that setting. My Yrth game broke down because one of the players didn't like some of the things that I used which were part of the setting. The biggest problem was a secret in the setting material: there's a tie-in to GURPS Infinite Worlds, which appealed to me, but since the players didn't know about that, it worked a lot like a bait-and-switch: the shared context wasn't fully shared.
Cidri sort of fell into the sketchy analogues approach, but actually it provided practically nothing for the players to work with; it was mostly just an excuse for why there was such a huge variety of stuff and vast unexplored territories: it's the world's largest planet! And there's a veneer of de-mythologized fantasy (vampirism and lycanthropy are diseases, not supernatural at all, for example.) And a science-fictional backstory that can be used to hang a secret on. It wasn't a very satisfying established setting at all.
So, there's the lessons I learned: use settings that the players can buy into and get context from, and don't use settings that have built-in secrets, especially if they go against the player expectations established at the beginning. Let secrets develop organically through play.