I don’t normally point to product comparisons since they date quickly and it’s easiest enough for most librarians to find these review. However, I thought that this one was particularly good (and I have an iPad so it’s a personal interest too).
Apple iPad Showdown: Battle of the eReader Apps
by Jason Perlow at ZDNet
Kindle for the iPad
Barnes & Noble eReader
Kobo Reader / Borders eBooks
I might just download the other three too.