Pros - It's open source. Easily installed from the Ubuntu repositories, which simplifies the installation of dependencies. A KDE GUI, KeyjnoteGUI, is available, which streamlines startup considerably.
Uses either a pdf file, or a directory of images - I've used the latter.
Transitions are very nice, though their smoothness depends on your hardware. Nicest on my Vaio laptop is "Crossfade".
You can script timings and effects in an .info file
A variety of neat highlighting and spotlighting effects are available while the presentation is underway.
Cons - No simple slide organiser
Would be nice to have a "next slide preview facility"
Update 20/01/08 - last Monday I gave a talk at a local school using KeyJnote. It worked pretty well, with pleasing crossfade transitions. The green "timer bar" that indicates where you are in the presentation starts runnign as soon as you set the presentation up. Since there was a delay in actually beginning the presentation, the bar wasn't much use (when I started the presentation, the bar had already advanced to about 10 minutes!). I don't know if there''s a way to work around this. I was presenting a series of jpgs, rather than a pdf document, which i haven't got to work yet. The only real issue was with the projector cropping the slides, but this is mainly due to the xrandr settings, I suppose.