I've collected Transformers for pretty much as long as there have been Transformers, and now have a collection of literally hundreds of toys. Folks who don't have collections probably think that's a little odd, but if you're a collector, this is actually fairly nominal. I know of several Transformers collectors, in particular, who actually have well over a thousand toys. I try to balance the needs of financial responsibility and fun a bit, and this is what works for me.
One of the first Transformers I ever got (that I still have, anyway) is also one of the more uncommon. It was obtained via an offer where you sent in several UPC symbols from Cookie Crisp cereal, along with $3, to get a toy of the Autobot, Jazz. At the time, I had no idea that "Cookie Crisp Jazz" was any different than "regular Jazz," but since I ate cereal anyway, $3 was more reasonable for me to spend than the $10 or so the regular toy cost in the stores, so that's how I got Jazz. Sadly, my Jazz is no longer quite complete: it's missing the missile that goes in the launcher on Jazz's shoulder. I also no longer have the little slip of paper with a black-and-white version of Jazz's box art and bio information (taken straight from the toy's Tech Specs that were generally found on the back of the box, but without the numerical statistics). And you can tell that some of the white plastic has yellowed a bit in the past 20+ years and that the stickers have faded. Ahhh, well. If only I could tell my younger self to take better care of my toys... (actually, in regard to the yellowing, it probably wouldn't have made much difference. To stop yellowing, you pretty much have to put toys in dark places and never look at them!).
I only learned that "Cookie Crisp Jazz" was considered a "variant" of regular Jazz in fairly recent times via the Transformers fan community on the internet. This site, no longer actively maintained, is dedicated to cataloging the variations in pretty much every Transformer made up until the time that the webmaster of the site decided he had other things to devote his attention to. On the site, you can see several pictures detailing what differences were made for the cereal promotion. Basically, the sponsor labels were removed from several areas, notably the windshield, the spoiler, the headlights, and the doors... well, at least that's what the variations site says. If you'll notice the pictures here, my Jazz indeed does lack the sponsor labels on the windshield, the spoiler, and the headlights. But my Jazz's doors still have the "Martinii" logos on them! I've never figured out what's up with that. Is my specimen unusual among "Cookie Crisp Jazzes"? There aren't enough of these around (certainly not cheaply enough today that I'd spend the money buying one), and even fewer that could be verified as having come specifically from the Cookie Crisp offer (I can't exactly document mine anymore!) that I've been able to find out.