What field are you in? I have extensive experience in PowerPoint and with switching over to Apple, I decided to pick up a copy of Keynote.
At first, I found Keynote easy to use by not "enough" for what I needed out of a presentation. I needed borning, scientific slides. However, a lot of transitions that are in Office XP are not included in Office X (I hope this is corrected in the new Office coming out).
Ultimately, I had to purchase a theme that was more professional for Keynote so that I could use it for my prelims. The presentation turned out to be excellent..but there were a few things I missed such as not being able to use subscripts and superscripts, my guidelines, and not being able to crop. Also, I found that my file was so large that during my presentation there was some serious lag experienced on certain slides. All in all, the presentation was graphically amazing compared to Powerpoint.
If I was not in a scientific field, I would say to go for Keynote. It is easy to use for a novice and has some fun themes which look like a lot of work.
However, here there are some reasons not to get Keynote. The exporting to Powerpoint was very poor, and if you need to give slides ever to another person (as I have to in my research group), you will have to find ways around this. There are a lot more features in PowerPoint...this might be why it is confusing for a novice. I just find that PowerPoint is much more powerful for a professional user, BUT Keynote can be equally powerful if you have the time (I guess the same can be said for Powerpoint slides).
The funny part, to me, is that alone each lacks, together, Keynote and Powerpoint would make the perfect presentation program. Each fills in where the other lacks and vice versa. Powerpoint is difficult, Keynote easy, Powerpoint technical but boring, Keynote less technical but graphically stunning....For me, I had to have Powerpoint for work and I got Keynote to satify my desire for graphically awesome presentations. But I am a presentation geek.