I have the new one, used the old one at my previous job for more that two years, have a MacBook Pro and Aluminum iMac with the wired version of the keyboard.
The aluminum keyboard is a gem, it's very easy to type on feels good and isn't that much of a pain to use for long periods. The only downsides I have with them are the low profile makes it hard to type A LOT of text for long periods of time, the tiny plastic keys will piss you off if you are used to the aluminum professional keys on the MBP, and putting any key cover on for protection makes the keyboard very NOT easy to type on.
I have key covers on my MBP and my iBook before it and they didn't render the keyboard useless like with the aluminum one. It's great without it, and since the MBP or MB don't have a number pad anyway it's a great companion when you are traveling. I use it for my mobile editing suite and it's a beauty, but the covers and plastic keys may start wearing on your feelings for it.
The previous wireless keyboard was the best Apple ever made IMHO. Since it was just the wired keyboard without the wire. The battery life sucked if you put anything other than lithium ion batteries in it, and even then they would die within three months. I am going on my third month using the new one ceaselessly and it only went down one notch.
If you can live without the number pad, and don't plan on putting any covers on it, go for the new one. If not, then there are 3rd party options and the previous Apple one as well.