I've recently started working on a presentation, about Ephraim Shay's diary during the civil war, and I decided to use Keynote 2
Keynote is part of Apple's iWorks Suite, which now just consists of a word processor called Pages and the presentation program called Keynote. There are strong indications that Apple is working on a spreadsheet program that will be part of iWorks in the future.
I don't do many presentations, and I've never used Powerpoint, so my experience is limited. So far, I've found Keynote to be extremely intuitive, and similar to Pages, which I've just been using for a couple of months. I'm pretty sure that Steve Jobs has had a lot of input into the Keynote design, because he said he's been using it for his own Keynote presentations for quite a long while before it's release.
Just like Pages, there are a lot of themes available, and I chose Parchment, as I am doing a historical presentation. Within a theme, you can choose from several masters, for each slide, to have bullets or pictures or whatever. You can add music from iTunes to any slide, drag & drop a photo onto a slide, and add transition effects when switching from slide to slide. I haven't actually hooked up to projector yet, but I'm using a notes feature that I can read from, that will only display on the primary monitor, while the secondary monitor only shows the slide.
Last Thursday, I started with a blank slate, and now I have 25 slides. More work needs to be done, but I don't feel like I'm spending much time learning software. I'm spending time digging up information for my presentation, as it should be.
One more thing, I'm becoming a very big fan of Wikipedia. Not only do I use it for research, but I also modify and improve things, as needed. What a wonderful invention...