I was happy with my existing 4GB iPod Nano, but always felt that an 8GB would be a much better fit for the amount of music I have. When I see the new release of the 4th generation iPod Nano, I just fell for the 16GB in Green.
The following weekend is spent converting all my remaining albums from 320kbps mp3 down to 128kbps AAC. Personally I think this is a fare trade-off between size (being able carry around all my albums with room to spare on such a small device) and quality (you can't tell the difference even with my Sennheiser earphones).
The iPod feels much thinner with its curved profile. The screen is also much bigger, shows full size album cover art, and features the new Cover Flow (album cover art scrolling) when tilted on its side. Sweet.