Debian Lenny might be called the 'testing' version but I find it stable enough to be used every day as your main OS.
Read the thoughts here
Installation was a breeze. I didn't try out the new GUI installer, Linux installation has reached a point where it's easier to install than any other OS, but seeing the old Debian text based installer brought me back to the old days where an early Debian version powered my first computer. Aaah, the memories