I just had enough time to sign-up for AdCenter today and upload a few keywords. The interface for adding keywords is a little clunky. The dropdown menu on the keyword page upload page showed me gibberish instead of options. Once the keywords were added I had to click on 50 individual check boxes to chose phrase match, and I then I had to click on 50 individual checkboxes to deselect broad match. Yuck!
It seems that AdCenter also relies on a Yahoo like approval process rather than letting you edit ads on the fly. I'm still waiting for my ads to be approved and go active, so I'm not sure if AdCenter does Google like A-B testing to automatically figure out which ad is best and serve it more. I love that feature in AdWords!
The day parting and demographic targeting look great, but it is going to take a few days to see if these features work as advertised. Everything is on hold for now, because as of 9pm EST I got an error message that I could not log into AdCenter due to an "unspecified data error." At 10pm EST I got a message that AdCenter had "no record of my user name." I double checked and it was definitely my user name. Hmm... not very promising, but I'll try again tomorrow.