One down side to CCL is that it will not run on Intel boxes that don't have the SSE2 instruction set extensions. That isn't a big deal if you're doing a web application on the server of your choice (which will probably have SSE2), but if you want to use CCL on a desktop application that runs on Windows (or Linux), it could be a very big deal. People still use old computers, and I believe some of the newer netbooks don't have SSE2. If you're doing a desktop app, the last thing you want your customer to see is "sorry, you can't run that on this machine".

You can read more about this on clozure.com:

http://trac.clozure.com/ccl/wiki/SystemRequirements

And that points to a list of SSE2-capable (and some non-SSE2-capable) machines:

http://en.wikipedia.org/wiki/SSE2

I bring this up only as a caveat to those thinking about using CCL. I think it is great that it is available as a free Common Lisp implementation. It would be even better, however, if this limitation didn't exist.

> that feature alone puts it head-and-shoulders over Python

Uh no it doesn't. Python uses OS threads. It cripples them with the GIL (hence multiprocessing), but it never used green threads.

And of course, using green threads can be an advantage if you're not dumb about it (see Erlang).

> it has a wicked fast compiler. So it makes a kick-ass webapp development platform.

What?