[nikkie]

iPhone: - Great because it is my phone and I always have it with me.
for reading on the go (at lunch at work or while standing in line). I use the following apps: Kindle, B&N eReader, Stanza. It is great for fiction.
For reading in bed.

iRex: - Great because it's size and weight is a good in-between. It's not too heavy.
For reading black and white manga with the CBZ addition by Kolenka
For reading fiction if I'm at home for the weekend during the day or on vacation. I don't enjoy it at night or in the evening because I don't like to make the room bright enough to make reading on it comfortable. I prefer my living areas dimmer at night.

iPad: - Great because of it's rich and large colorful screen.
For reading technical books (ePub or PDF) with tables or anything that's long and needs careful study and/or notations. I use iBooks, Kindle, and readMe. I'm still waiting for the B&N eReader to be updated.
For color comics/manga.
For playing games of all sorts. It is a far more enjoyable experience than on my laptop. Particularly fun for Scrabble, solitaire, go, and simulation games.
For watching Netflix in bed.
For downloading and viewing/playing sheet music.
For reading the internet on the couch or in bed.

Each device is useful in its own way. I may end up selling my iRex just because I use it less, but I've grown more attached to it than I expected due to its excellent size and balance. I'm not in love with eInk by any means. The screen is quality but I find the contrast unpleasant, though better than its eInk competitors. A normal book page reflects considerably more ambient light than an eInk screen. In bright light it is excellent. I suspect if Apple released a 7" iPad, I would trade my iRex for it. The regular iPad is a bit heavy for my wimpy-girl-arms.

The iPad has a lot more potential than it uses right now. I don't read on it as much because the only ePub app that supports calibre server and has a night view doesn't have a two-page horizontal layout, which I prefer. If you prefer to read black on white, I'm sure iBooks will serve you well. You will most likely want to turn the brightness way down while in the app though.

One note: I'm not reading technical books on my iRex because the selection from B&N is pathetic, and I did not have a good experience freeing my Kindle books. Amazon has a much better collection, which limits them to my iPad.