Well, unfortunately I have not been able to play with many in person, just the OLPC XO and the Sony Reader. I was basing my opinion on researching them online.

The ones I've looked into whose design I preferred to the Kindle were the Iliad, the Sony Reader, the Hanlin Reader, and the OLPC XO (in e-book mode).

It's not the same category, but there are also dev kits available from e-Ink that are supposed to be suitable for building a prototype e-book reader.