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.