Well, well, well. As it happens I have been researching this myself. I have over 300 ebooks on the computer and spend hours reading. I have begun to hate setting up the laptop to read, so started looking at Ebook Readers. Here is my humble opinion ...
Stay away from Sony. Everything I have heard, with the exception of the reply on this forum, has been negative. I just looked at WalMart, which carries the Sony, and was disapointed that it neither had WiFi nor 3G. Now, that may have been an older model, I really don't know.
I set up a Nook, from Barnes and Noble, for a friend but it seemed slow and limited. Some of the menu functions were screwy and it just struck me as not to 'user friendly'.
I am looking for the best deal on a Kindle. It comes in 3 varieties. All support about 7 different types of Ebook, including PDF, and there are 'converters' you can use on your home computer to get a compatable format if the one you want is not supported. ie. Microsoft .lit is not supported.
The WiFi Kindle is about $130. Has a 7" screen and looks user friendly. If you are near any WiFi hot spot it will connect to Amazon and you can browse and download books. There is a FREE library available with thousands of 'out of copyright' books that you can download.
The next Kindle is the same size, has WiFi, but also has 3G connection through Verizon. No additional cost or subscription. This version costs about $180 and also has a 7" screen. The last version is a Kindle DX and is exactlly like the 3G version except the screen size is 9.7". All of the Kindles have a new screen format that is suppose to 'look like' paper, be easy to read in bright light, and have several fonts to select from. All the Kindles also rotate the screen if you want to view something in 'widescreen' rather than 'landscape". For me, it looks like the best deal.
Hope some of this helps. Good luck ...