For the past few weeks I've been borrowing a first generation iPad, I've been using it everyday at home as my laptop was stolen. I also have my own kindle 3G. I just finished the book "The Fires of Heaven" by Robert Jordan; this is the sixth or seventh book I've read entirely on the kindle.
Reading pdfs on the kindle is possible, I read one full book this way but it was far from ideal. The kindle uses a very good text layout algorithm when it has the raw text, this is impossible with the portable document format which results in either seeing the whole page in miniature on the kindle screen or seeing thirds of the page when viewing in landscape mode.
Buying books for the kindle is so painless. The 3G model that I have gives me access to the kindle store in places as remote as Arthurs Pass, and purchasing a book is but a few clicks away. On the off chance you have other ebooks in other formats the kindle has support for quite a range, or the software Calibre will convert between most formats. I had no trouble loading the few.mobi books I had onto the kindle.
On the iPad front, I have to say I was a fan as soon as I saw Steve unveil the thing. As a news reading device, a map, a twitter front end, as a general web surfer, you-tube player, music player, and as a recipe book in the kitchen... it is absolutely superb. I haven't tried an android tablet and I imagine they are everything the iPad is without the apple lock in. The free app "Flipboard" is freaking fantastic. Now that I have my own laptop again... who knows where the iPad fits in. Something used to flick through the news in the morning, and the twitter/facebook feed over a coffee?
For reading technical reports, power-point presentations and the like; the iPad wins hands down. For reading novels with few pictures the kindle is a perfect book replacement. But be careful to charge either of them before going away, I had a brick of a kindle coming back from Kaikora when I forgot to charge the battery in about a month. Two very cool toys!