I switched from 505 to the Kindle 3 a couple months ago and haven't looked back.
CON
- My Kindle 3 in its lighted case *is* a little bulkier than the 505, but not so much so that I notice.
- I like the Sony's homepage a little better.
- I can't set up collections on in Calibre for the Kindle (must be done on the Kindle).
- Must buy a Kindle case separately (it was included with the 505).
- The pagination is weird on the Kindle. It uses "locations" instead of a simulated page count. That makes it less than intuitive if you try to jump back, say, ten pages.
PRO
- Not only is the screen better (higher contrast IMO), but the typeface is hugely better. Italics actually looks like designed italics as opposed to slanted Times New Roman (again, IMO).
- My Kindle doesn't crash. My 505 does, frequently.
- I've found Kindle's instant dictionary look-ups to be surprisingly handy.
- Kindle doesn't "reverse text" the footnote number or symbol. Not a big deal, but I always found it distracting on my 505.
- I can charge my Kindle with the same charger as my cell phone and bluetooth headset.
- I love how the Calibre news function drops my Instapaper articles onto my Kindle homepage.
- It's so much faster.
The only thing that would stop me from switching is if I had a big DRMed library, but this wasn't an issue in my case.
You have no idea how much I loved my 505, but I haven't looked back since I made the switch.