Full review
In shopping at Magazines.com, I've found several magazines I didn't previously know existed, and I now read them regularly and love them. I have also been able to renew quite a few magazines that I already subscribed to either through the magazine/publisher directly or from other online places to purchase subscriptions like Amazon - but now with the majority of my subscriptions at Magazines.com, I can keep track of everything all in one place, which is a great convenience.
They offer an easy renewal option, which I know some people (including some of the previous reviewers here) don't like, but the way I think about it, if I'm going to renew the magazine anyway, it's nice to be able to "set it and forget it" and know that I won't miss an issue. The few times I've needed to cancel a subscription or have had any other customer service issues, they've been handled swiftly and courteously both through their online contact services or through their 800 number (800-MAGAZINES, which is displayed prominently on their site - another plus, since most online stores don't make it obvious how customers can contact their customer service departments).
The only negative I can think of is that their site is a little dated-looking, and quite frankly, pretty ugly, but from what I can tell, it's secure and reliable, and I've never had any trouble doing what I've needed to do.
In all, I've been very happy as a customer with Magazines.com.