As per this study done by the mobile usage firm Onswipe, the general usage is predominantly landscape. To quote the article
Mobile usage analysis firm Onswipe, as part of a slideshow celebrating
its second year of operation, revealed some further analysis of user
data from iPads that in some cases reveal interesting habits and in
others reinforce facts already known from other end-user studies.
According to the analysis drawn from its 127 million users over two years, iPad users prefer landscape to portrait,
The article further states that the type of usage is dependent on the content being consumed
Many games and other apps also prefer to use landscape mode, though
web surfing and book reading tend to prefer the vertical orientation.
Users have said that due to the keyboard being wider in landscape as
well as case stands tending to orient it that way, landscape is also
the preferred view for typing.
Interestingly, this informal survey from Brooks says that the portrait orientation is preferred by most users.
I ran a little survey to gather a few data points on iPads and their
usage. I tried to make the survey fun enough that users would quickly
answer the questions I really wanted to know, without a lot of thought
put into them.
Look at this split for the normal orientation that an iPad is used in:
That’s pretty astounding when you think about the fact that
PC/Desktop/Laptop manufacturers all but ditched this type of view,
thus preferring the widescreen view portal. What’s even more amazing
to me is the user preference for the iPad’s orientation:
That said, he makes a very valid point that users will their iPad in the orientation which best suits them and the choice of orientation would be dependent on the application and the use.
Users, by-in-large, use the iPad in whatever way they see fit for the
task at hand — not in line with their screen orientation preference.
This interesting article about Portrait or Landscape highlights that only 21% of the top 200 apps in the app store support both orientations (at the time the article were written) and hence the orientation preferred would be the orientation supported by the app. To quote the article
Here’s how the 200 top paid apps look.
Surprisingly, only 21 percent show off their app in both orientations.
Despite the primacy of portrait in Apple’s iPad marketing, the
majority of top paid apps use landscape in theirs, at least within the
App Store. That’s no guarantee of course that this is how people spend
most of their time on their iPads, using these and other apps.
Not all categories are represented equally in top paid apps, which
happens to be extremely game heavy. By looking at each App Store
category individually, a better picture emerges of the diverse ways
that iPads are held and used.