"Power corrupts. PowerPoint corrupts absolutely." -Edward Tufte. (OK, that essay applies just as much to the alternatives as it does to PowerPoint, but the quote was too clever not to use.) To be honest, I've never used presentation software enough to have much to say about it. PowerPoint's obviously a pretty good program, but like many Microsoft products, its dominance owes more to Microsoft buying it from the original developers and then bundling it in Microsoft Office than any uniquely compelling features. There's very little it can do that the "also rans" can't do, and there are more than a few things that the alternatives can do better.
Apple Keynote represents the current state of the art in this category, with a so-far unmatched combination of ease-of-use features and advanced support for multimedia, including scalable graphics, loads of smoothed fonts, dual displays, etc. It imports and exports all of the file formats you're likely to want. The main down-side is that it's only available for the latest Mac OS X. Other than that, it's priced affordably.