Our group is currently running a Middle-Earth campaign. Set in Third Age 2800, and using Iron Heroes rules. Our spellcasters are using True Sorcery with the arcanist variant from that book.
Very action-oriented (thanks IH!), and our group (now approaching 8th level) is coming to terms with just how powerful an arcanist can be. They actually fit into the world quite nicely, with one conceit: you understand that they and the Istari are playing in two completely different ballgames.
It's been a blast! There is alot of wilderness in Middle-Earth and all the things that can happen to you from point A to point B are fun to address. Of course, some enemies get a redressing to fit with the setting, but do a quick scouring of the many Tolkien compendium/excyclopedia sites and you'll see there's actually alot of creature variety that the casual movie-watcher or even someone who just ran through the Lord of the Rings books didn't pick up on.
We also entered this campaign with the express knowledge that what we did COULD have far reaching consequences. For instance, if the characters did for some reason go and actually manage to slay Smaug (who at this point has only been at Erebor for thirty-something years I belive) then there would be no Quest for the Lonely Mountain in Bilbo's time, hence he doesn't find the ring, hence alot of other things change.
I should mention by the way that our group has played high-level, super-magic, low-level, several systems and editions between all of us, and we have ceased our "Tim's campaign this week, Jim's next week" campaign hopping we usually do. We have stuck with this for a good two and a half months now without anyone mentioning wanting to try something else on the side.