For reading only, iPad is better and worse. However, iPad handles non-Kindle based formats better, like .doc, PDF, etc.
I would hate to read on a iPad for any length of time. I get eyestrain reading off both an iPhone and an HTC Desire due to the backlit nature of their screens. The electronic ink on the Kindle is brilliant and I can read it for hours at a time, just like paper. OK, you cannot read the Kindle in the dark, but then I cannot read a book in the dark either.
(This is from someone who always scoffed at the thought of reading on any electronic reader than on paper.)
Install Calibre (
http://calibre-ebook.com/) which converts between most ebook formats as well as managing your complete ebook library. It will also handle sending the book files to your Kindle (via USB or email).
Brilliant for reading fiction. Loads of now out of print books are available on newsgroups/torrents and loads of free ones in Project Gutenberg and other free archives.
My main criticisms of the Kindle are
1) the appalling way it handles a large library; you cannot simply get a list of authors and then browse to the author you want and display his books to select from.
2) you cannot add series data to the books, so if you have a series of books, you have no idea what order they should be read in. I have started changing the title and adding a numeric prefix, but I really shouldn't have to do that
3) it is not good at books that need layout. Textbooks and other books with large images/tables can cause problems.