I have a few objections to that, as nice as it is for what it's trying to be.
The first is covered by the "Is there any kind of Âpresenter screenÂ in Impressive?" FAQ. (Or more directly, the "No, and there is currently no proper way (or plans) to implement such a thing" answer.) There's sort of a half-assed workaround that gets some of the way there, but a half-assed workaround is still half-assed.
The second is that I don't think PDF is a good delivery medium for a lot of presentations. A lot of people (especially here) will decry things like fancy effects and animations, and when used without purpose they're distracting and obnoxious. However, they can also be used very well, to clarify relationships or show how a system transitions from state-to-state and stuff like that. I get the feeling that PDF is a bit more capable here than I give it credit for, but I still think it's pretty poor in comparison to something in the PPT/Impress/Keynote genre.
Third, it's only a viewer, which leaves open the question of what you author the PDFs in. The example slides are Beamer, and as much of a fan of Latex as I am for documents*, I think it's a pretty poor fit for most presentations. Partly this goes to my previous point, but I also think that presentations are a medium that minimizes most of Latex's strengths and maximizes its weaknesses.
(* Actually this is untrue. I hate Latex. :-) But like PPT, it stands out as being by far the best of a bad lot.)
Like you say, to each their own, but I think it's not for me.