You could ask on comp.lang.lisp if you wanted some really well-informed opinions wrt the impact of Arc on CL, but I wouldn't advise it unless your skin is thick and flame-resistant.
http://groups.google.com/group/comp.lang.lisp/browse_thread/...
> MzScheme installations with unhygienic macros and empty lists that
> serve as false?
http://groups.google.com/group/comp.lang.lisp/browse_thread/...