And I can't think of much to add, so I'll start with linking Nick Roche, a UK fan who's become involved in the writing and drawing of comics in a pro sense after an active youth in the fandom scene over here...
I'm not a particularly hardcore fan, by my reckoning, although I suppose having contributed in a limited way to Official Stuff marks me out as a bit more obsessive than most people. And it's having a head for trivia that draws a lot of people towards the franchise, which is replete with hundreds of characters just from the eight years or so that was the initial phenomenon — there's a degree to which it fulfils the same impulse that makes people collect baseball cards, or learn dinosaur names, and other OCD things. Categorisation spins out of our basic ability/need to identify and often just plain invent patterns.
The toys have never really fussed me that much. The fiction, on the other hand, lit my imagination up as a kid and continues to every so often still. Alien robots that turn into cool things and fight each other... what more could kids want? Some of them turn into dinosaurs, too. In theory there's scope to tell any kind of story with broadness of the cast and setting. It doesn't always turn out like that, of course. Part of the compromise of the fiction is that it represents a major toy manufacturer and as such can't get too far onto more mature topics, such as Bad Things that happen in a war that's supposedly lasted longer than most humans can realistically conceptualise. Plus robots are functionally immortal and shouldn't, if we're aiming for a shred of internal consistency, make some of the stupid plot-convenient mistakes their all-too-human writers ascribe to them. But on the whole Hasbro has let an awful lot of things slide — robots can be seen to die on the page or screen in ways that the cast of the A-Team or of He-Man can't. You can have characters who're cannibalistic serial murderers as long as not too much attention is drawn to the fact.
The imagination factor came largely from the fact that, with so many characters to stuff into fiction to promote the toys, very few of them got much character development. Instead, kids would read the short bio cards printed on the back of packaging and extrapolate from there. Inconsistencies in portrayal between those bios, the comics and the cartoons only added more potential for us to fill in the gaps and argue about which bits worked best. People are still doing that a quarter-of-a-century later; later Transformers series and comics (because the franchise has never really stood still in terms of churning out product) have never attracted quite as much discussion and interest, and I think a lot of that's down to a relative lack of inconsistencies in, say, Beast Wars.
But I'm already waffling. Thanks to Bob Budiansky and Simon Furman for creating most of the characters I give a toss about, to Hasbro for taking an enlightened attitude towards fan sites and fan fiction/art, to James Roberts for writing a riveting unofficial self-published novel that's helped further demonstrate the potential Transformers has to be 'proper' science fiction as well as kid-oriented toy promotion, and to all of the mad bastards I've talked to about TFs online over the past ten years or so.
Here's to more stories.