and they love them. A big part of it for them is they live in Trinidad and they have a hard time finding books, and what few books they find are really expensive (used books there cost more than new books in the states). They also travel quite a bit and really like having all their books in one small package.
I tried them out for a few weeks (they were shipped to my house before their visit in the fall). I quite liked them, but decided not to get one as I plan to get a rev 2 iPad.
As Mike points out, the iPad can read books from anybody:
iBooks app
http://itunes.apple..../id364709193?mt=8
Nook app (Barnes & Noble)
http://itunes.apple..../id373582546?mt=8
Kindle app
http://itunes.apple..../id302584613?mt=8
Borders app
http://itunes.apple..../id368067444?mt=8
and more - do a search on "ereader".