For some reason no WYSIWYG tool ever works for me, be it for writing texts or generating presentations. My workflow simply doesn't match the one of those tools, I end up spending too much time thinking about the look rather than the content.
In addition I like my work to be under version control and versioning big binary blobs (and gzip compressed XML isn't really better than that) isn't that cool. So I prefer tools that work on plain text.
Some time ago I tried S5 and liked it, except for the fact that I couldn't create a PDF of the real slides instead of the print view S5 provides.
Then I stumbled over the LaTeX Beamer Class and used it for an internal presentation I did this week. For somebody familiar with LaTeX this is really really great, I can only praise it. It is an "apt-get install latex-beamer" away from Debian/Ubuntu systems and I didn't have any trouble getting it to work on my Cygwin pdflatex installation at work either.
This is the example directly from the class's homepage
\documentclass{beamer}
\usepackage{beamerthemesplit}
\title{Example Presentation Created with the Beamer Package}
\author{Till Tantau}
\date{\today}
\begin{document}
\frame{\titlepage}
\section[Outline]{}
\frame{\tableofcontents}
\section{Introduction}
\subsection{Overview of the Beamer Class}
\frame
{
\frametitle{Features of the Beamer Class}
\begin{itemize}
\item<1-> Normal LaTeX class.
\item<2-> Easy overlays.
\item<3-> No external programs needed.
\end{itemize}
}
\end{document}
which produces this PDF. In the end I had to select a nice theme and things looked better than anything I ever did in PowerPoint. And it certainly took me less time, was version controlled and all that.