Since my interest in Lisp was first piqued by Graham's essays [1], and I'm reading On Lisp too, I was naturally curious about the dialect he designed. I liked what I learned in the tutorial [2], and after getting a taste of Arc's syntax, the relative verbosity of most other Lisps was not very attractive to me. I know this may sound superficial, and it is (literally), but as shader said, "a few bits of syntactic sugar...go a long way." Some reasons aside from aesthetics I'm still using Arc: I've been able to get things done with it, it's well-tailored for making web apps (which are my primary interest at this point), and I've found the community (though small) to be especially thoughtful and helpful.

I recently stumbled upon a post from about a year ago in which the author explains his preference for Arc over other Lisps [3]. It might be the perfect thing for you to read given you're looking for "why people here use ARC over CL or Scheme". One of his points aside from brevity is that macros are especially writable in Arc, which is a big deal considering they're one of the most powerful tools Lisp has to offer. It's also reassuring to know that the author (aw), who was then so enthusiastic about Arc, continues to be a top contributor to this forum and maintains the resources at http://awwx.ws.

One of the interesting things I've realized about this community is that not everyone here uses Arc over CL or Scheme. That is to say, it's not just a bunch of incestuous Arc ideologues: a lot of the people are exploring other Lisps and other languages too, e.g. elibarzilay is into PLT Scheme [4], thaddeus is learning Haskell [5], waterhouse has been implementing Arc in both PLT Scheme and CL [6], jazzdev and conanite have used Java to write Jarc [7] and Rainbow [8], respectively, etc. This is an indicator to me of how open-minded they are, which IMO is a critical trait for any community to have, but especially one so small as Arc Forum.

Finally, just a logistical detail that may help you get started in Arc: version 3.1 does work with the latest MzScheme [9]. This was a point of confusion for me after reading the install page [10], and while not a big deal, it can make installation a bit less of a pain.

Hope this helps. :)

[1] http://www.paulgraham.com/lisp.html

[2] http://ycombinator.com/arc/tut.txt

[3] http://catdancer.github.com/thank-you.html

[4] http://barzilay.org/

[5] http://arclanguage.org/item?id=10952

[6] http://arclanguage.org/item?id=11529

[7] http://arclanguage.org/item?id=9084

[8] http://arclanguage.org/item?id=6002

[9] http://arclanguage.org/item?id=10254

Brevity is a major one; a lot of arc code is shorter or cleaner than equivalent scheme or CL code. Fewer parenthesis, more logical names, a few bits of syntactic sugar that go a long way, etc. Just compare the code in ac.scm to arc.arc, and decide which you would rather write, or read. (not quite fair, since ac.scm is mostly compiler stuff, and arc.arc is just the basic library of functions, but there is still a noticeable difference in readability and brevity)

I also like arc because it's really easy to change to be the way I want it. Since the whole language definition is less than 2500 lines long, half of that in arc code, and very few functions more than 10 lines long, it's easy to figure out how it works, and easy to change.

Libraries aren't really a strong point of arc, but using Anarki (the community version on github) it's pretty easy to just use PLT's libraries, which are quite extensive. Speed is the only other real "con" to using arc, and I haven't really found it to be much of an issue.

As for tutorials, PG wrote an arc tutorial that is on the main arclanguage.com page. Other than that I don't know of much that has been written specifically about arc. arcfn.com has a lot of nice tips and a decent documentation section, though there isn't a link to that on his front page.

Other than that, SICP and On Lisp will go a long way towards understanding arc. If you have any questions about the differences between arc and CL or scheme, or just questions in general, feel free to ask.

Most of us here are somewhat disappointed by the current lack of activity on the arc forum, and would gladly welcome and answer whatever questions you may have ;)