> it's so powerful that it's bad
I had the pleasure of using Cascalog in production at work, which is written in Clojure. While it was in fact written by some very smart people, we had a very difficult time using some of its constructs that were very cleverly abstracted away behind macros.
The problem was that it felt nearly impossible to debug problems we had because of the long, impossible stack traces. Further, trying to get another very smart programmer to understand why some functions behaved in one way and some behaved in very different was very hard to convey. I'll reiterate that I thought the other guy I was working with was really smart, and I'm at least not an idiot, and we both felt like we had a really hard time unwrapping what the code was doing.
On the flip side, if it were written in Java (I think some parts are actually but more under the hood), you could point at the code and say "That's where the map function gets called on all the workers" (Cascalog is for Hadoop), or run the code and get some kind of stack trace where you could even begin to start figuring out what was going on. We weren't even doing anything cutting edge.
For me, I love the academic/fun endeavor. I have wasted countless hours playing and learning. But if you asked me if I would base any critical part of my production app on Clojure, especially when there are more than a couple people who weren't Lisp experts, I would have a really hard time justifying it after what I saw when I tried.