I don't have experience with the Treos, but I think we can assume that the screen is similar to the Palm TX that I currently use for reading eBooks.
The main issues I have with the Palm is the direct sunlight issue and the screen size issue. Other than that, it works nicely. If I am indoors, without my sunglasses on, reading eBooks on my TX doesn't cause eye strain or anything.
But I can say that, after reading eBooks on my iLiad for the last 2 days, reading eBooks on my iLiad is FAR nicer and what I plan on using for my eBook reading from now on.