There is currently a war going on between Amazon (Kindle) and Google to win the e-book market. Google's main weapon is deliberately making their entire library incompatible with Kindle, but compatible with everything else. Kindle, OTOH, already owns the lion's share of the e-book reader market. Betamax vs VHS all over again, I guess.
I happen to use a Kindle DX my wife gave me for Christmas last year, and like it a lot. However, it was expensive compared to the new generation of Kindles, which I think are only around $130 or so for the same capability as the DX. Pretty reasonable.
Preloaded books should not be a consideration, since (assuming you buy the 3G wireless capable version of Kindle) there are a zillion free books available directly online that will download in seconds to your Kindle the minute you click on them, and use of the 3G wireless downloading service is free after you register your new Kindle with Amazon. So many, in fact, that the hardest thing is finding time to even look at them all online to select what you want to download. There are of course a zillion non-free books available, usually for less than $10, but I've downloaded very few of those so far. With a pocket paperback at WalMart going for $8 a pop, downloading a recent best seller hardback for $10 begins to seem like a bargain if you like to read a lot.
In addition to Amazon's Kindle book store, there are many free downloadable books on line on several websites that you can download to your PC and then dump to the Kindle via USB cable. Just about anything written in the past 200 years that's no longer in copyright (because of years elapsed) is available for free somewhere online.
I like my Kindle, and some of the best books I've read on it were freebies written in the 18th/19th/20th century and therefore out of copyright. Also most of the classics are available free at Amazon's Kindle book store.
Admittedly the only device I have experience on is the Kindle, but it works great and once you've paid for and registered the device with Amazon, there is a nearly endless supply of free stuff, and an infinite supply of paid stuff you can instantly download and read. A 500 page book takes about five seconds to download via 3G. Very slick. One of these days they'll have a color Kindle, which will be a big seller, I'm sure, but the current version is really wonderful for people who love to read but don't want to pack around a stack of books. That's me!