Slideshare is great, and from the start provided clear value (youtube for powerpoints). So why are there no Slideshare competitors (where there were heaps of Youtube competitors)?
Slideshare is ok but I love using Prezi(http://www.prezi.com) for my presentations now. It allows me to make presentations that are really interesting and that keep people engaged. Since my audience tends to be developers/testers this can be difficult but having things move around without be "Death-by-powerpoint" movement its a winner!