I'm going to have to (somewhat) disagree with the replies here. I have an early unibody macbook pro 15". I've upgraded it to 4 GB of RAM, and a 7200 RPM 500GB drive. The rest of it is default specs (as if there was anything else you could change)
I also have a slightly newer Macbook Air...with the standard 2GB RAM and (this is key)...the 128GB SSD drive.
I ran a few simple benchmark tests (and I'lll see if I can find the results...I'm pretty sure I saved them)...this MB Air wasn't quite as fast as the pro...but it wasn't as far behind as I thought it would be. (and I had my MB Pro set to all "high performance" settings...no power savings settings)
This is, no doubt, a result of the SSD drive. If you are going to get a MB Air, make sure you go with that option...it will cost you a few hundred bucks more (which is typical Apple marking up the price...SSDs aren't cheap...but they're not Apple's price of upgrade either). Unfortunately, the SSD in the MB Air isn't a standard 2.5" laptop drive form factor...so you can't upgrade to one yourself.
Either way, I bought my Air and since then have turned on a few folks to this machine. And they absolutely love them. I'm talking guys who are power users...guys who aren't necessarily playing games on these machines...but who want performance, nonetheless. Particularly those who were torn between a regular PC laptop, a (higher performance) netbook, or a MB Pro. Obviously, Macs cost more no matter what....so I wasn't going to convince anyone who couldn't afford more than 12-1300 bucks, tops.
But if you've got the cash...and are willing to put down $1700-$1800, you can't beat the combination portability and performance... Once again..SSD drives only. Otherwise, I'd skip on the MB Air.