I've been using Debian testing for a while, and here's what I have to say about it

Debian testing

Advantages

• Plenty of packages in the repos
• apt is very fast
• Uses sudo as default if you tell it to
• No need to edit any configuration files
• Modular KDE
• Flash works on 64bit!

Disadvantages

• No AUR or anything similar so I have to compile packages the old fashioned way (./configure, make, make install)