If you've read Geoff's piece at GameSpot on The Final Hours of Half Life 2
then you already know that quality level of the new app. If you're into developer commentaries, have an interest in Valve's design process and own an iPad give this app a whirl. I think it was well worth the $1.99 and from reading the feedback comments at the end of the app most people who also purchased it feel the same.
The interactivity is really something else with there being lots of pictures, user polls, panoramic pictures of office, slideshows and sound clips. All the interactivity combined with Geoff's 15,000 word BTS feature on the the last 4 years at Valve really make this a great package. You learn more about the employees that aren't the usual Valve public faces and there is an interview with Steven Merchant.
After the success of this app I really feel that a behind the scenes apps is going to become the new trend amongst major releases right along side the current trend of having a video production co (2 Player/Area 5) do behind the scenes videos.
Buy the app here: http://www.thefinalhoursofportal2.com/
Favorite Pictures (just a few don't sue me Geoff):
The animators of Portal 2, most of which are ex-Pixar:
The Narbacular Drop Team (sans Kim Swift):