I have a profound love for the Wilderlands that has been little dulled despite the fact that I haven't run a campaign there in quite a few years. To give you an idea of how young I am, the 3.5 version by Necromancer Games was the first thing I bought entirely with money I had earned. I still own it, though the box is a little worse for the wear, and occasionally I glance through the pages and have a deep sense of nostalgia.
I think there is more to it than nostalgia of course. It's one of the most versatile settings, as Scott and Rob Conley's blogs can attest. At the same time, at least for me, there is a feel to the material. It's some sort of mishmash of the Hyborian Age, Lankhmar, and the Lord of the Rings that represents the full wackiness that so embodies the unholy concoction that is D&D. At the same time it's palpably different from a more Gygaxian setting. It's always seemed more like the Mediterranean c. AD 500 (or possibly later) rather than the 15th Century assumed setting of TSR D&D. It then has a light sprinkling of the Classical world and even some Persian stuff added to the mixture. It's a world where the paganism of D&D makes 100% more sense, and -- at least in the 3.5 products -- it uses that paganism to the hilt.
It has a wide variety of civilizations, cultures, and therefore ruins. They have rough equivalents to actual cultures (such as Skandiks) that are pretty easy to detect and therefore easy for players to grasp. The gods are a wonderful mix of real and fictional ones and most are the definition of awesome...
Well, to be honest I doubt I have to convert any of my readers over to the Wilderlands. Its reputation among Old Schoolers is well known.
Throughout most of college I was running the Wilderlands in some form or other. My players have adventured in Lenap, Tarsh, The Elphand Lands, and of course the CSIO. It is the CSIO campaign I remember most fondly as it was the one that both lasted the longest and... well is most memorable. I co-DMed with another player (Brandon who comments on these posts occasionally and has a blog about video games called WannaDev) and the players stole sacred stones, fought in arenas, were mostly slaughtered by bears, led attacks on Skandik outposts, scaled the Tower of the Elephant, met a "juvenile dragon" (an ancient worm who liked fart jokes), and even witnessed the destruction of the CSIO by a giant Markab robot. The crowning moment was when my (now) wife rolled three 20s in a row to attack said robot, thus killing it instantly and gaining enough xp to ascend to godhood.
The science fantasy aspect has been one that I've always liked and that I've emphasized or demphasized to various degrees over the course of my DMing the setting. One time the players were in the Ghinor Highlands and met a group of Duregar who kept Derro on leashes and used crackling purple laser pistols (I like "crackling purple" as my description of ray guns).
Recently my thoughts have turned to it once again as my ennui with the Dark Country continues. I had started developing my own version of the Wilderlands c. 2008 (I found out about this whole Old School thing from Scott's Wilderlands OD&D blog), and I might use it if my campaign kersplodes. Who knows. I just needed to reminisce a bit about a great setting. Theres a lot of adventure in those maps.
In fairness I should also link to JP's Death Blog: The Blog that Eats People, a blog for horror movie reviews and commentary. He's not only another friend, he played Bjorn (of the Bjorn Horn) in the above mentioned CSIO game.