Amazon's Instant Video for iPad offers a diverse library of pay-per-view TV shows and movies, plus an additional subset of "free" content for subscribers to the company's $79-per-year Amazon Prime service. But it's important to note, that you can't purchase content from within the app and will need to visit the Amazon Web site to purchase or rent TV and movies.
With that caveat, we found that Amazon Instant Video is an effective rival to iTunes if you're willing to do a little extra work. There are a number of differences and advantages to each service, but Amazon has definitely closed the gap more than ever before.
Amazon's Instant Video store (on the Web site) has all the major studios and networks just like Apple does. Even so, availability of content is a mixed and mysterious bag, as it is with most online video stores these days. Certain titles appear on iTunes but not on Amazon, and vice verse. Meanwhile, TV content on Amazon and iTunes has normalized, for the most part. Most TV shows are available on both services. The bottom line is, both services offer plenty of movies and shows, but have holes in their back catalogs; check through both to see what's available and missing.
Pricing might be the biggest reason to try Amazon Instant Video. So far as we know, TV and movie studios set the prices on their content, so most Apple and Amazon pricing will be identical. But Amazon seems to offer lower prices on many titles, presumably eating the difference as an inducement to get more business. Amazon also only offers movies in HD, so there's a one-size-fits-all price for rentals or sales. Rentals range from 99 cents to $3.99, and movie purchases tend to range from $9.99 to $14.99, but you'll occasionally see discounts down to as low as $4.99. Most TV episodes cost the same as they do on iTunes, but prices for the rest of the content are either lower or match iTunes prices.