The new 7 speed transmission is interesting. It improves fuel economy with no performance loss, but you do have to get used to things like double gear drop downs and the fact that the car runs at about 2000 rpm vice 3000 at typical freeway speed. The new electronics suite is greatly improved over 2004, but the harmon/kardon audio isn't quite as good at the surround processing as the Bose system in the 2004 or as loud. the iPod option works really well--way better than the 3rd party junk that's out ther.
Dropping the top with the keyfob is the best feature of this car. The integrated v710 cellular phone works astonishingly well, and the iPod integration is awesome. Get a nano so vibration won't damage the storage.
There's no longer anything to improve about the CLK!