I subscribed to Prevention for a while.... magazines seemed to have waaaaay too many ads in them, and sometimes the info in the mag seemed a bit suspect. So I quit subscribing.
What I like about Spark is that the info here is scientifically sound. It's not the latest fad or whatever's hit the bold type on the covers of magazines or onto Dr. Oz or whatever. Let's face it, the magazines (and Dr. Oz etc.) are in business to make money. Spark is free. There is no hidden agenda in the articles or the advice from the experts on this website.
I think with those "21 days free" offers, they're hoping you'll miss the window of opportunity to return the book, and therefore have to pay for it. I'd bet their book isn't much different than Reader's Digest's book or any of the myriad others out there.