I was having a problem viewing pdf files on my Firefox 3.6.3 on Ubuntu 10.04 (Intel Core2Duo with 2 GB RAM) everyday and faced rendering issues while trying to view a PPT prepared on MS PowerPoint in my Ubuntu Open-Office recently! I also knew about Google Docs Viewer... And so one thing led to another, resulting in this add-on! :)
My full list of reasons of why you might like and use this add-on:
1) No more painful pdf reader loading in your browser (resulting in your system slowing down, browser hanging or totally crashing!!)
2) Lighter on the RAM, so that your browsing experience doesn't suffer.
3) Super-fast loading time, thanks to the Chocolate Factory guys (a.k.a. Google Engineers)!
4) Don't regret that you clicked on a link, ever!!
5) No rendering problems with MS Office documents (Word and Powerpoint), if you are an Open-Office user by choice on Windows/Mac, or are an Open-Office user by default (read as a Linux user)!
6) No more need to download to sluggish PDF readers to view PDFs in your browser
7) ...and many, many more!