I haven't had to give a presentation since library school but I have been subjected to plenty. For the most part they are boring, look exactly the same, and the presenter hands out copies of the slides which always made me wonder why we even all gathered together in one room. Why not just email us the slides rather than standing there reading from them?
The value of presentation tools Prezi and Slideshare aren't that they will magically make your presentations better - because only you can do that - but that they enable everyone to share. Sharing means inspiration and learning, finding awesome presentations and using what you've learned to make your presentations better.
In terms of capability, Slideshare is primarily a place to upload presentations you've made using PowerPoint. Prezi is actually a tool for creating presentations, and it allows zooming and panning and other non-linear movements. Like PowerPoint, the presentations could easily all look alike. But if you spend some time looking at the presentations on both of these sites, you are sure to find some great inspiration and advice on how to create a powerful presentation.
Do you ever create presentations? What tools do you use?