Keynote is an amazing slideshow editor. I prefer it over PowerPoint on the Mac. On the iPad, it works beautifully when creating complex slide transitions and element effects, which competing apps can't do. And its iCloud syncing can be a real life-saver. A bonus is Apple's free Keynote Remote app for the iPhone and iPod Touch that lets you remote-control a Keynote presentation on your Mac or iPad.
Keynote's big negative is its awkward requirement of copying documents to and from cloud storage services, rather than allowing direct access as other apps do. But it supports AirPrint and PDF export.