I don't run the retro-clones, preferring to run the games they're based on (e.g. OD&D or AD&D), but I'm a solid supporter of them. I don't need them for rules, but I appreciate them as a means of providing new support material for the games I play. I can go down to Nan's Games & Comics and buy OSRIC modules that I can use in my game. I also appreciate that the retro-clone and "inspired-by" systems are bringing more gamers and attention to my kind of RPG market. I love having magazines like Knockspell and Fight On! around.
While I personally don't run a retro-clone (although I'd really like to run Mutant Future at some point), I do see their value as games to run, too. A few years ago, I gave my eldest son a copy of the Mentzer Basic Set. He loved it, playing the solo adventure, making his own dungeons, and running friends and family (including me) through the stuff he created. He was also very hard on the aging books. When it came time for him to expand beyond levels 1-3, I didn't really want to give him my old copies. I thought about a Rules Cyclopedia, thinking a hardcover would hold up better, but they're pretty pricy unless you get lucky, and finding one in robust shape for a low price requires *real* luck. (Also, I simply can't stand the RC interior art -- blech.) However, a hardcover copy of Labyrinth Lord was just the ticket, and my son is running that, now.
the main appeal to me seems to be nostalgia and a harkening back to earlier, simpler, times - when dungeons were endless and monsters random.
To me, the appeal is my preference for the older editions: their look, their feel, and most importantly -- the way they prep and play.
It's not nostalgia or rose-colored glasses. In my case, it's not even the appeal of the edition I started with, because I found that works the best for me is original D&D (1974) -- and I started after that, around '78 or '79. While I think the rules are simpler, I don't find retro-clone gaming to be "simpler" in the sense of being less sophisticated or less "grown up." For example, I'm a fan of "endless"
mythic underworld dungeons. But I don't think dungeons should be limited to that model, and I don't think that such a dungeon need be completely senseless or random. Speaking of randomness, I'm also a great believer in the utility of random rolls. I like random encounters (e.g. wandering monsters) and their effect on the game. I also make use of random rolls for monsters and treasures when I'm creating dungeons and adventures, as well as picking and choosing. I see randomness as a tool and a springboard for creativity. I don't feel bound by my random rolls. Instead, they are nudges or suggestions, sometimes stretching my creativity in directions that I might not have gone, otherwise. Randomness keeps things from getting stale. I find that to be true with PCs, too -- I usually prefer random rolls over point buy because I like the experience (and challenge) of "seeing what hand fate deals" and making something fun out of it. It's often surprising how a sub-par or "hopeless" set of rolls ends up being a beloved character (perhaps a PC I wouldn't have even tried, otherwise).