I hadn't looked at PLT Redex since its pre-publication days, but yesterday I started using it to implement my draft model for Topsl, the programming language for implementing online surveys. I couldn't believe how easy it was! I expected it to take quite a while, but within an hour I had a working operational model of core Scheme, and half an hour later I had the entire grammar of Topsl and even a few nice abstractions. I haven't quite finished the implementation yet, but it's been surprisingly painless.
Of course, it helps to have a good understanding of macrological* pattern-matching and template instantiation, capture-avoiding substitution, and Felleisen-style context-sensitive reduction semantics, so you might say I'm uniquely suited to use the tool. Regardless, I'm very impressed.
* This is a very silly adjective. Please don't ever use it. Someone could get hurt.