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