Well, on the design side, Arc is great, and I really love hacking with it. I'll keep doing it whatever happens. I'm even one of the few (I guess) users of the language for "real" work. The application I wrote a few months ago is still in use and she's fine, thanks for asking.
However, as it is now, it is really not a practical language : the implementation is slow, bugged, impractical (do you have the right version of mzscheme ?) and doesn't provide useful error messages.
I know, I know, this is a feature, not a bug. It will be ready in about a hundred years. It is not supposed to be a language "du jour" as, say, Python et al, addressing the trivial problem of solving today's needs. Well, that's great, but I do have needs to solve today. So, as for now, I'm using Lua or Python for real work.
Hence Luarc I already told about, which I'm using now for webapps. It doesn't have all the commodities of Arc (or even other Lisps), it even hardly has macros but, guess what ? It has libraries and can communicate with the world. I can even read from databases ! Can you imagine that ? Oh, I know, this is so "21st century", but I can do it ! It has all I really need : closures, dynamic typing, easy and fast development, apps written in a continuation-passing style, etc. I would like more, but I already have that.
Arc is for the next century ? Well, OK then, see ya in 2108. And I'm sure I'll be using it everywhere then, not only for my hobbies and for esthetical/philosophical reasons. If I'm still alive, of course.