a. Have you found it rewarding or is it something you wouldn't do again?
It's great to get law-related experience in any form you can get it. And you may get a good reference out of it. And, you become more informed about the issues, even if you don't develop a deep knowledge. I would volunteer again.
b. What were the biggest challenges you experienced?
The biggest challenge for me was getting assignments that weren't essentially clerical work. Legal aid and public interest firms don't have the money to hire the support staff they need, and it's easier to assign mindless work than to train and supervise inexperienced people who won't be around full time. By talking with my supervisors, I got research and writing and client interviewing tasks that were more interesting.
The projects where I had these problems involved litigation. Working at the brief advice clinic may be a lot better because you could get some great client interviewing experience and perhaps also some client counseling experience. Depending on your background, it may increase your understanding of the legal needs of the poor. I doubt you'd learn a lot of substantive law, but that's what law school classes are for.
c. Have you found it helped or hurt you in your job search? As in, do you avoid talking about it with firms or do they generally value the experience?
It's on my resume, but the firms don't seem to care much about it in OCI. My experience, however, did contribute to my performance as a summer associate, and I expect that it will help me as I seek out pro bono opportunities once I start work.