I just went from the 08 alu macbook to the 2012 11 inch Air and let me tell you, its night and day.
I had the 2.4ghz alu macbook configuration and later upgraded the Ram to 8GB and put in a 1TB drive. After 4 years it still handled my computer needs (all be it a little slowly sometimes) so I didnt really need to upgrade, however I felt id got enough 'value' out of it and would treat myself to a new machine.
I ultimately chose the i7 11 inch Air over the 13 inch model as I was wanted something ultra portable. I'd say im not a power user and mostly use my machine for email, media and a few professional applications. The i7 Ivy processor and more importantly, SSD with Sata 3 connection meant it was more than capable of handling anything I could throw at it.
The instant loading of apps, 'instant-on' from sleep and cold boot time of around 13 seconds makes a huge huge difference to the computing experience - everything is snappy and there when you want it. Its heaven.
I recently used my old trusty alu macbook and the thing felt like a tank! On top of that it was unbearably slow compared to my new Air.
If you have the means then do it. Certainly not a necessary upgrade (for my computer needs at least) but I'm extremely happy that I did.