I bought a Sony PRS505 for the same reason, and had the same doubts.
But reading from an e-book is much easier / comfortable than reading from a screen, or printing the documents (that you maybe only read once).
It is true that some pdf's are not rendered correctly, but in that case I play a little bit with the zoom level.
There are also a lot of tools to make the device even more useful (reading blogs...)
Sony has now a newer model with touchscreen. I you want to take notes or search in PDF's, you'll need this model. Otherwise take the PRS505, cheaper and a clearer screen. The overlay for the touchscreen causes sometimes some reflection and makes it less clear.
Conclusion: very happy PRS505 owner...