It sounds like most people here are talking about paying markets (and science fiction, specifically) but I submit literary fiction, so it's a little different. New online journals for lit fiction are sprouting up everywhere. To be honest, now when I hear about a new online journal (that doesn't pay, of course), I kind of sigh. I'm all for new publications and I'm sure some are capable of being really great, but overall it's so easy to start a new online journal, and so many people are doing so, that I'm a little wary.
I did submit to a rather new online journal (run through a university) and I was accepted. It is an attractive journal and the editors are great about drumming up publicity for it, so I have had a good experience. However, I have decided that was the last time I would submit to a new, non-paying online journal.
This isn't an online-vs.-print thing for me. There are some really lovely online journals, even ones that don't pay, that I would love to appear in, but otherwise the market seems diluted with these new places that don't pay.
If the journal pays, or its creators have demonstrated that they are putting extraordinary care, time, money, or attention into the new journal, then I would be more likely to submit.