I've been using Boo quite a lot lately. It's very flexible and very powerful. The metaprogramming works well, but it's not nearly as easy to use as Nemerle's. In addition, the lack of arbitrary expression nesting (e.g. def foo = if(bar) match(baz) { ... } else 0;) makes certain things harder than it has to be, but that's not something you're going to miss unless you're coming from Nemerle, OCaml, Haskell, or something along those lines.
Overall, I'd say give it a shot. I don't think you'll be disappointed.
