It takes things even further though, as it has a Z dimension so can zoom your audience around in 3D. You can created an entire presentation in a single HTML page, style things, play video, show images and link to other sites.
It's a great system, and did I mention it's free? What it lacks, however, is a simple way to layout your presentation as you have to supply x,y,z coordinates for all of your presentation steps. There are some projects alive on the web workin to solve this - such as Strut which are develping nicely, and again free.
In some ways, using Powerpoint/Keynote is easier, but in other ways doing everything in an HTML file can be simpler and faster. It depends on the presentation and what the format of the presentation is, and who will need to edit it. You do need to be pretty comfortable with HTML to use Impress.js, but if you are - and don't need presenter notes etc, then it could be a great alternative.