Unlike word processors and spreadsheets, presentation software doesn't have a single industry leader to compare to. While Microsoft Office PowerPoint probably has the largest install base, Apple's Keynote is far ahead in terms of features and quality. Unfortunately, as with the rest of Apple's iWork suite, Keynote only runs in OS X on Apple hardware. Therefore, we will not be using it as a comparison product.
And, as with spreadsheets and word processors, presentation apps were tested using a .ppt test file that I found online. Note that I did open up this category to include apps that do not support .ppt files. As a result of mobile devices and the advent of bootable flash drives, those giving presentations shouldn't worry too much about compatibility with any proprietary format. For that reason, backward compatibility was not considered mandatory.
OO.o Impress (v.3.0.1)
Among the presentation apps for Linux, OO.o Impress is the most closely related to PowerPoint. It imported the test .ppt file flawlessly with transitions, objects, and all. Impress also tailors your presentation to overhead projector transparencies, traditional slides, screens, and even paper, furthering its usefulness, independent of medium.
If you don't already have PowerPoint, save yourself some cash by using Impress instead. But jump back to reality here. Being on par with PowerPoint is really just a tie for second place. Any shop serious about presentations no doubt has an Apple lying around for running KeyNote. Bottom line: Impress is really the only viable presentation software for Linux, if coming from PowerPoint.
SoftMaker Presentations (2008 Trial Edition)
SoftMaker Presentations opened the test .ppt file, but it confused numbered lists with bulleted lists. This is by far the most lacking app in SoftMaker's Office suite, both in terms of compatibility and feature set.
SoftMaker Presentations has a paltry number of options compared to PowerPoint or Impress. However, it is much more user-friendly than the other apps listed here. You can add text, tables, or pictures into any box other than title boxes simply by clicking the corresponding icons within the blank object boxes. I can see how somebody who doesn't need complicated slides or hasn't used PowerPoint in many years would get by just fine with this app.
KPresenter is the only presentation application that does not open MS PowerPoint files. While that's not a deal-breaker, its lack of template variety is. This app is not user-friendly, and if you've spend time in PowerPoint, you will not feel at ease with KPresenter. The layout, functions, and steps needed to create a new presentation in this app seem alien in almost every way.
Compounding its user-unfriendliness is the document structure sidebar, which breaks slides into their various components (text, tables, pictures). While this could come in handy for large documents, as it does in KWord, it seems totally unnecessary for single slides.
One positive that I noticed was the crispness of text and objects when played in full-screen slideshow mode. Unless you're planning on running KDE and using the rest of KOffice, pass on KPresenter, especially if you're used to PowerPoint.