The KREO kits are really nice for making custom transformers. Of course, there aren't enough joints to make a toy transform, but the main sets are still surprisingly versatile. Pictured here, among others, are G1 Megatron (from the Megatron kit with approx. 10 additional small pieces) and Cheetor (from the Bumblebee kit).

This version of Megatron has a passable fusion cannon and a reasonably G1 physique. He's also quite tall - possibly as tall as the Optimus Prime set, though I don't own it - thanks to a partially hollow torso that keeps him from being top-heavy. I added a few windshield pieces to the chest and back to make it seem like he transformed into something, perhaps a truck. I'll upload a back view later if I can, since he looks pretty good from the back.

Cheetor is really poseable. As you can see, he can shake paws with Bumblebee, chew on hapless Kreons, or get ready to pounce! I used two Kreo foot ball-joints in the body articulation to help matters, one for the waist swivel and one for the tail. He also uses small swivel-joints for the paws. I can't help but think that he looks a little more like the Sabretooth Tigerzord than Transmetal Cheetor, but I can't deny that he turned out well.

There are a few other customs on display, including a fembot that looks to be a cross between Elita-1 and Crasher (the Go-Bot), and a velocoraptor also created from the Megatron set. I prefer Cheetor and Megatron to the others on display.

As for those wishing to create their own custom Kreo kits, I'd say out of the three kits I have so far, Bumblebee's is by far the best kit and the most suited to customization. Why? Bumblebee has the most "detail pieces" (flat plates, sloped pieces, itty-bitty translucent pieces) of any of the kits $30 or less, along with the most "redirection pieces" - plates allowing you to join sections of lego at right angles. Furthermore, the base Bumblebee robot mode blows away the base Megatron or Sideswipe robot modes, as it's very poseable and actually looks like it transforms thanks to some ingenious torso detailing.