I was 5 years old when TFs came out and once I became a fan I was diehard. Nothing else came close, and those that stood a chance were other transforming robots (Voltron, Machine Men).

I think the key thing that hooked me was the aspects of creativity and imagination. The toys are a puzzle but they were also characters in way toy cars or generic robots weren't, and that made them more 'alive' to me. I still call them he (or she) when I think I should be calling them "it".

As a kid I'd study the toy catalogues and work out in my mind how each toy transformed (some in later years I was right about and some I was wrong on). I'd draw Transformers, write stories about them, play with them, play as TFs at school lunchtime, and create my own characters by imagining how things could turn into robots.

And many years later I still do these things - well not much playing as TFs but very occasionally I'll might out a transformation sound when standing up.

The comics kept me going longer than some of my childhood friends, because they were stories I could revisit in a way I couldn't with the cartoon (and the UK comics were great!) and when the comics stopped I kept imagining the continuing stories. I still find the expanded TF fictional universe so diverse and interesting that even though I'm extremely well versed in it, I can't get enough of it.

And I still enjoy getting a new toy and seeing how it converts. And if it's a toy with vague characterisation I enjoy giving them characters or doing a digibash or two.

tl;dr:

Transformers inspire creativity and my imagination and that's why I'm still a fan 30 years later