I've to admit, I haven't used PowerPoint for 4 years, mostly because no one wanted me to present anything. Times have changed, I had to do 4 presentations in one week lately. Being forced to communicate through slides, I experienced a bit, and here's my conclusion.
The first presentation was built using Keynote, which is part of iWork, Apple's Office counterpart. Creation was really simple, the look is just amazing ( especially the transitions ), but there is one problem: I don't have a mobile Mac at the moment, so I needed to use a friend's MacBook to do the presentation. This is definitely a bit of a pain, considering the fact that he's most likely not always around, and exporting to PowerPoint kills all the USPs. There is a workaround though, you can export presentations as .mov files, movies that halt on every transition and continue only after you press a key.
Second presentation, Latex Beamer producing PDFs. The ugly part is the creation, certainly. Unless you are fluid in a text editor, command line and the reading of technical documentation, this might not be your premier choice. After you've gracefully managed to create some slides, the output looks great and the best point about it is that any netbook will do just fine for the presentation. I used my Ubuntu-equipped machine and it worked well using the default PDF-Viewer.
Sven showed impressive to me today, a great way of tuning your PDF-only presentations by adding transitions, overview pages and other handy accessoires to finished PDF-Presentations. It's written in python, has some dependencies ( nothing that easy_install couldn't fix ) and works really well.
The third presentation kicked off using Google Docs. The presentation ( about Google Go ) had no style, no transitions, was built in about 10 minutes and.. worked. Nothing more, nothing less. It clearly did it's job, yet there are, at least by now, a bunch of more pleasing ways to get the job done available.
Because a lot of people are really excited about Prezi at the moment, I decided to give it a shot. The presentations created are really different from what presentations have been in the past, it looks.. amazing, is easy to use, features a free plan. It depends on flash, which is somewhat a drawback and is, at least for me not a choice at the moment. Why? Well, first of all, there is no way to create custom styles, you are basically limited to what Prezi offers. The next problem is the lack of a free offline editor ( Google Docs is able to do that ). And of course, what you create looks create, but either I missed the feature that says "copy region from one Prezi to another" or it's not there. Either way, without it, reuse seems impossible. Still, if you have only 3 minutes and need to deliver a stunning performance, you should go with Prezi. ( Edit: Benny just posted something about Prezi a few days ago, too )