There's no point trying to improve on something that works well, doesn't weigh too much and lasts for ages. The 2008 rear mech looks for all the world like its predecessor, mainly because nothing has changed.
The Centaur rear mech features a carbon composite outer link plate, which is nice but doesn't save weight or increase stiffness hugely. It's more an eye-candy statement for the carbon-tarts among us.
One nice thing is this mech can be serviced properly; there's no faffing around trying to degrease it then lube it, you can take it apart, completely. This, coupled with excellent durability, means you'll have this mech for years unless you crash it.
Shifting is quick and precise. A lot of people rue the day Campagnolo introduced its quiet shift Ergo levers, but the only real difference is that less pressure is needed to shift and it isn't as noisy - and the rear mech is a tight and precise unit. If you're going to run the big 13-29 rear sprockets make sure you buy the medium cage version; anything under and short cage should be your choice.