I think it's 'meta hackable' in that you can choose what 'hackable' means :) Some people may choose "last release 4 years ago" to mean it's dead. I choose to think 'stable release' doesn't apply to arc. Even PG's versions have the disclaimer "future releases can break anything", which doesn't seem very stable. So I and a few others choose to "break anything" of our own accord :)
'Hackable' isn't black or white, but a question of priorities. SBCL and Racket care about keeping the implementation hackable, but have other higher priorities: it should provide a stable interface that doesn't break user code, it should provide a compiler, it should run programs really fast, it should help teams of people work on large projects, etc. With arc, hackability is the #1 priority. It's a pretty good platform for experimenting with language features, or at least a pretty good forum for discussing language features you build on any platform. But it's not the ideal place if you just want features and don't care how they're built.
