I recently replaced my HP PDA and my cellphone with an
HTC Touch Pro 2 smartphone. It is Windows-based, which I felt would make it easier to transfer most of my PDA apps and files into the smartphone (which it did). That app transfer included most of my e-book-reading apps, though those that are not touch-based do not work so well as others (for more detail on that,
see here). Most importantly (for this site), the screen quality makes e-book reading a breeze.
Bottom line, I love it. I note the same as others, that smartphones eat battery when being used in their smart, or "PDA" modes... but most allow you to switch off the PDA functions by putting the phone in "standby" mode, giving you battery life comparable to other phones. My solution: A charger at home and at the office, where it largely stays connected when it's there.
My wife got a Motorola Droid before I got my TP2. I can only comment from over-the-shoulder comparisons (she mostly won't let me near it, for fear of what apps I might load on it and scare the bejeezus out of her!), but the combination of functionability and available apps makes it a great smartphone, too. Personally, I'm glad I got the TP2 instead.