Sounds like an awesome encounter. I'd have no problem setting Materials and Mentals against my groups (and they can attest to that hehe), but I was cautious not to go too far with the Clobbermob's immunity. I figured I'd try and give a taste of the old school flavor, but not make it too frustrating for a casual group. If it can serve as a base for other DMs to tailor to their style or difficulty, though, then I count it a total success.
These are great. My favorite minion I've made was a simple ice archon attendant minion that deal a tiny amount of cold damage in a burst when it died, but more importantly it made all the ground within a burst 1 of it into icy terrain. Gotta love a minion that can make the floor slippery when it dies. There's nothing that makes an encounter more fun than a lot of heroes falling down.