I write for Suite101 and Examiner, and am still considering whether to take the plunge and apply to Demand. I'm in the same boat as you--I work 30-40 hours a week at another job, and so I carefully consider how much 'pay-off' I'm getting for the time put in during my writing time, such as 'Is it meeting long term goals?' 'Is it writing that is building a business for the future?' Also, I'm writing a novel and have another fiction project going, and though I don't think those will ever really pay off--I do them b/c that's what I love--I want to make sure I allow/make the time needed to pursue those goals. Eventually, I hope the non-fiction will pay the bills. In 2-3 years, I'm hoping to have built a substantial enough business to go full-time, weaving togther web writing and print and also possibly books and/or teaching.
So, in light of that:
Suite101 seems worth the time/effort so far. I like it b/c I get to choose what topics I want to write, and therefore build a portfolio and gain experience writing about the topics I want to write about. I use some of my Suite articles as writing samples when I apply for online writing work. Also, I've already learned so much there in terms of SEO and web writing. I've only got 10 articles there so far, but I'm already seeing some profit, and I can see how it can snowball and be a good deal in the long run. I like to think of it as a 'future bank account.' As it's been said, Suite is the slow-and-steady tortoise approach.
Examiner, I like b/c I can also write about topics/subjects for them in which I'm very interested, and it gives me further experience with web writing. Plus, I get some exposure here, in my city, and I think that will come in useful at some point. Plus, I see the pennies building to dollars at a rapid pace, so it will pay-off sooner rather than later.
I keep considering Demand...I hear it can be good money. The only thing that makes me unsure if it is a good match for me (and my particular goals) is that it doesn't seem like a place where one can 'build a business,' ie, build a steady returning client base. I'm thinking I might rather put my efforts into doing that, as long as I have the advantage of income from another job. People kind of look at Demand as 'dirty work,' ie, you do it for the money. Not something to get clips or references out of, or even referals. But, it's good to know it is there, and I know many people are making good money there and are happy with it.
Anyway, that's my 2 cents.