So far I'm impressed with OpenOffice's presentation tool (called simply: Presentation I think). I find it easier to use the PowerPoint, and I don't see any reason to use power point in the future.
One area I found to be lacking was the pre-built presentation templates, there are only a few to choose from. But don't worry OpenOffice still has all the annoying features, such as animations, and transitions!
The coolest feature in my opinion you can export your presentations to swf's (Macromedia Flash), or PDF. That's nice since to my knowledge there are no free ppt viewers for OSX (that I have found anyways). You can open PPT files with OOo, but you can't them with OOo. That's not a big deal though, since swf is a better vehicle for sharing presentations on the web IMHO.