I am not a heavy SlideShare user (yet), but I like the service very much.
One reason is that you can easily share your presentation (even with PPT download) with a large audience and also publish it in your blog.
Also you can browse other presentations on all sort of topics through categories or search and that way you can often find what you need.
When going through the service today I found that they also provide something that was new to me : slidecasting. A combination of your uploaded presentation and a podcast, how cool is that …
So on my todo list for the coming weeks is to mess around with this service and create a slidecast 🙂