I do. Google Chrome and Safari both open PDFs very smoothly on Windows. Even in Firefox I am not sure Scribd is an improvement over opening plain PDF (it shows a progress indicator, but it might as well take more time to open in total).
Be rational. Most users use IE on Windows. Even if you don't like it, that's the way it is. Adobe Reader 9 loading in IE can crash the browser. Don't believe me? Google "IE loading PDFs" and you'll see the huge number of users who have problems.
Flash is reasonably reliable in IE. Additionally, Scribd has the option of embedding ads in the PDF more easily by using the player, which gives them a path toward monetization.
When creating a business, you have to be realistic. This is not a techno-utopia where the best technology always wins. In the browser wars, IE is still the current leader. You have to live with it if you want to maximize profit.
Then you should have said "Internet Explorer opening PDFs is a pain point and scribd solves that". Fair enough, although I can argue Chrome is both much better solution to this particular problem :)
Personally I think scribd solves some problem for publishers, while actually degrading the experience for a significant minority of the viewers. Their relationship with publishers have significant negative externalities, and this makes scribd at least somewhat evil.
iPaper on the Mac breaks the very clean PDF viewing that's built in. That's kind of sad because breaking things on a Mac are really hard, but Flash does it.
On what kind of system is iPaper better than Adobe Reader 9?
Adobe Reader used to be really slow. But, even on my almost-three-year-old laptop, Adobe Reader 9 if quick.
I honestly don't know why Scribd doesn't do something to improve their product when obviously so many people hate it. Scribd is one of the very, very few things about my computer that still frustrates me.
In general iPaper does a good job for smaller documents (press releases, etc.) that you just want to skim. It sucks for very complex documents, or for reading longer books online. It was never designed to be a replacement for Adobe Reader -- that would take years considering the pdf spec has the same magnitude of complexity as HTML/CSS.
Also I'm not sure what metric of 'quick' you use. The iPaper swf is around 150 kb, and the Adobe Reader binary is around 30 MB. Take a look at http://www.dearadobe.com/top_rated.php - most of the top five are related to Reader bloat.
