Amazon Kindle

I performed this test yesterday as it happened- reading Quantum Thief on my Nexus One/Kindle.
Train journey to Canterbury- Nexus One from full, off charge at 07.15 was totally dead by 12.45.
The Kindle's battery is meant to be rather better than half a day.

I use Aldiko mostly with white text on black and screen brightness down. The longest I've used it though is an hour's Tube journey so I'm not sure how long you could get out of it.
The point is I'd have a phone no matter what, whereas this would be another tech item I'd have to carry around.