Zinio is not the best app, but it does have the most magazine subscriptions available. Not everything that you can get from their website is available for the iPad. Some limitations are due to Apple's mature content restrictions. Others are format. Zinio offers books, but only the magazines can be downloaded from the iPad.
On the plus side you can download and read the same magazines on both your iPad and computer, and only pay once.
My biggest gripe is the time it takes to download a mazazine on the iPad, and how much space they take. Other than that, it's an ok experience.