Google Presentations, which is free, is part of the company's online office suite, Google Docs.
Right off the bat, you will notice that Presentations has some of the same basic functionality as Microsoft's PowerPoint. It does enable you to create some really basic presentations, with themes, but the lack of features and slide show polish are real turn-offs for me.
Yes, there are nice collaboration features, just like the other Google Docs applications, but if the final product isn't on par with what PowerPoint produces, those features are almost irrelevant. The omission of basic animations and transitions really take away from it.
PowerPoint-style presentations have two major objectives. Those are to inform the people to whom you are presenting and to hold their attention. I would personally not feel great about using Google Presentations on an important presentation, where I need to impress people. The presentations that it creates just do not have the "wow" factor.
Despite all of that, the collaboration features are really the service's strong point. Not only can other people collaborate on the same presentation, but when you are done, you can either share it via a public URL or present it to a group of people that you invite. This is really where Google gets it right.
Google Presentations is a decent free, Web-based solution for creating slide shows, but the limited feature set hurts it when compared with PowerPoint. I give Google some points for the collaboration and sharing features, but that's not enough to get me to switch. I understand the concept behind trying to provide a simple solution, but this is a case where simple is not necessarily better.