Some answers - Edit 1
Before modification by Tavi at 29/07/2010 07:27:45 PM
...and the trees sing 
I'm sort-of maybe thinking about getting one, and have two questions:
- What are the pros/cons/USPs of all the different readers?
- Are ebooks a standard format that are compatible with all readers, or if I buy a certain reader am I locking myself into a certain ebook format?
If you want a full comparison, let me know, and I'll write something more detailed. Here's the basics, though.
1) Depends on what you're looking for.
Price (readers): Amazon is a little cheaper than B&N, and they're both cheaper than the Sony equivalent.
Price (books): Amazon, B&N, and Sony typically sell the e-book versions of NY Times Bestsellers for ~$9.99. Outside of those, Amazon typically has a lower price point.
Features: Amazon has more features than B&N or Sony. B&N offers chess and sudoku, Sony has a full touch-screen reader. Amazon has password protect, folders, popular highlights, share passages on twitter/facebook, the list goes on.
Internet: Amazon's 3G works internationally, B&N and Sony are US only. They all allow transfer of documents from your computer.
Formats: Amazon doesn't support ePub, which is (apparently) one of the more popular formats for electronic documents. B&N and Sony do. Amazon offers more format options without conversion than either.
2) You're not locked into a particular format by buying one e-reader over another. There's enough conversion software out there that you can pretty easily convert from any format to another, as long as there's no DRM on it.