My wife just got an Amazon Kindle eBook Reader as a gift so I have been setting it up and poking around it this Saturday afternoon. Love the e-ink - it is so much easier on my eyes. I normally get a ton of reading done on my iPad (with the backlit screen) but it stresses my eyes out. For purchasing and reading books, the Kindle seems so much more soothing and much like actual paper.
I sent an email to my Kindle email address (each user gets a unique address - ****@kindle.com) with a pdf attachment. I could see the pdf on the Kindle but the text was so small that it wasn’t wortht he squint - so, I am guessing that people use the email attachment option only in emergencies cause it is not usable on a day to day basis.
I am an avid Instapaper user so I set it up to sync wirelessly with the Kindle. It worked great but I couldn’t find a way to delete individual Instapaper articles on the Kindle, there is no two-way sync and no way to email out the article or push it to social networks. When I read something good, I like to have the ability to share it with people that matter to me.
Hence, for articles I am sticking to Instapaper on my iPad.
For books, it is Kindle, provided she is not reading at the same time :)