All of the comics in Marvel Unlimited are at least six months old, to avoid competing with the day-and-date sales of new comics as they get released. Those are available through the Marvel Comics app.
Gizmodo tested Marvel Unlimited out, and had positive things to say:
Having played with it a bit, the HTML5 reader isn't the fastest or most responsive, especially on pinch-to-zoom (which you can see in the video), but it's mostly acceptable. Still, it's a long way from the smoothness of native apps like the Comixology Marvel app, Kindle or iBooks. Marvel said it's a "trade off for having the same reader and experience in the app as you have on the web." It displays in very high res (via progressive download), even when you haven't downloaded the full book for offline reading. Marvel couldn't confirm if the resolution is the same as the retina-level Comixology books, but they looked tack-sharp as a full page.One of Gizmodo's biggest complaints was that users can only download six issues for offline reading, not nearly enough to read on a plane flight, for example. A Marvel spokesperson said the company was "listening" to reader feedback on the subject. He also noted that this is "phase 1" of Marvel Unlimited, and more would be coming later this year.
The comics available through MU are all at least six months old, and while Marvel wouldn't commit to every title making it into the service, SVP of Marvel Digital Peter Phillips told us most major titles "should be in there at six months; we're pretty timely with that."
Marvel Unlimited is a free download for iPhone and iPad from the App Store. [Direct Link]