I gave the app a try with an HDR presentation I did a couple of years ago. The upload was 5 MB so it wasn't much of a hit on my free storage. I sent an invitation to a friend, and checked it myself in my browser. The presentation looked fine. My animations held up, and colors and fonts were the same as on the native file. SlideShark allows you to draw on your presentation, and use a virtual laser pointer. I could also skip ahead or go back. My remote presentation followed by a couple of seconds.
A few caveats. You aren't going to get audio narration through the app, so you'll have to use FaceTime, Skype or a speakerphone. Also, only embedded WMV or AVI video files will work. There is a presenter mode so you can see notes and timings. The app also supports AirPlay, so you can connect to an AppleTV for people who are local to you.
The Pro version is an in-app purchase, and offers one year of 1 GB storage for US$94.99. If you are constantly using PowerPoint, this is a terrific little app. Most people can get by with the free version. It's secure, as online presentations require a password to log in. Reviews for the app are uniformly positive.
The app is universal, so the iPhone, iPad and iPod touch are all supported. SlideShark is optimized for the iPhone 5 and requires iOS 4.3 or greater.