Here's a tip that helped me when I was shopping for an eReader! -- know what you want it for, because with a sea of endless features that vary for each model, you'll need to know what's important to your reading experience to be able to make a decision.
Is buying books on your device very important to you, rather than using a computer? Do you want to play games on it? Or an mp3 player? Or do you want to be able to read different kinds of formats on your reader? How about organize your books into series and genres or favorite authors?
Know what you need before you go browsing.
Then... YOUTUBE is a great place for window shopping. There are tons and tons of video reviews on almost ALL the readers available. You get to see how they look and work. Where the memory card slots are, how fast page loads are, and hear experiences from the reviewers. Also, there are comparison videos so you can see models side by side.
And ask around. If you've narrowed it down, start asking questions on forums about peoples experiences with the readers you have in mind. Google the product name of the readers along with words such as 'problem' or 'broken' and 'bad service' etc... so you can go into your decision fully loaded with all the information you need.