Working for free is something that comes up often when you are beginning your career. There are mixed answers to this question on the internet. My answer is: "it depends". It depends who is offering the job, and what they will be doing with the artwork. If the person offering the job is someone anonymous on Craigslist and is offering you exposure as "payment", I would say no to these jobs. They don't really care about you or your artwork and are just looking for free work. If the person offering the job is an art director at an already established magazine, for example, Discorder, this is a little more credible and you can accept these jobs if you are looking for experience working with an art director. These kind of jobs will also result in portfolio pieces for you. If a friend wants you to do free work, I think this is a personal decision you have to make. Basically I think you need to ask yourself some questions:
Is the project interesting?
Will you get a portfolio piece out of it?
What kind of exposure do you get?
Will it be good experience (i.e.: working with an art director)?
My first magazine illustration gig was with Discorder, a local Vancouver magazine. It didn't pay, but I got some portfolio pieces out of it. I also learned how to work with an art director, and how to work within certain restrictions (i.e.: black and white, spot illustrations). I also learned how to work under a deadline. Working on a real life deadline is different than working under soft deadlines in school. There were no real consequences if you didn't get the work done on time. For me, those first non-paying jobs were worth it. Paying work followed, and I had more confidence going into those jobs knowing I had some experience already.