I've tried several different tablet PCs and have been reading ebooks since around 2003.
My various reading devices have been:
Tablets:
TC1000 >
Motion Computing 1200 > Motion Computing 1400 > Fujitsu P1610 >
TC1100
PDAs
Cassiopeia 110 >
Ipaq 1910 > Ipaq 1945
I have a Fujitsu P1610 and while I think it is great as a small and portable laptop I don't find it wonderful for reading for extended periods. The up and down controls make turning painful for reading for longer than about 30 minutes. Also, the weight gets annoying when reading at night. I often read novels for an hour or so at a time so page turning and weight are important.
The best device I have for extended periods is my Ipaq 1945. It is small and light enough to enjoy reading on, has a battery which lasts for around 4 hours and supports all the various ebook readers. I have several spare batteries which are useful for long flights etc. I don't use it as a PDA for appointments etc, just for reading (novels, non-fiction and bible study). I have found that a backlit screen is essential and the very light weight of these devices makes them easy to carry everywhere. I traveled for about 3 years and found the size to be perfect I also brought one for my wife who never thought she would give up paper books and after a novel or two she got used to it and now won't give it up.
For PDFs, reference books and and general tablet PCing I use my TC1100. These can be bought very cheaply secondhand and have a fantastic form factor for reading. I would buy one of these over the P1610 if I was choosing between the two.