I have tried on and off over the last few months to get to grips with Prezi. It's been picked up by management at work and use encouraged. However, I still feel I've not seen anything which has really made me think 'wow!' over using Powerpoint.
Whilst I do run training sessions, most of my training is either one-to-one or based around active use of internet resources, so I don't really even use Powerpoint a lot now.
I can see the advantage of Slideshare, sadly my only experience of it being used in college was to find it was blocked!