Performance:
Let me put it in this way: 2 years ago I still had a P III @ 450mhz 512kb cache (YEAH!!! 450mhz!!!!) 384mb ram and at some point i had "upgraded" it to a nividia 4000 graphic card with 64mg video. That machine served me for over 8 years (1998-2005) with at most 2-3 upgrades (graphic card from a S3 savage 16mb which dont think it exist anymore, and 64mb ram to 384mb and a Maxtor Diamond 10 with 120gb from a 13gb HHD!!!!.) Now this is the point where you ask, what kind of program you could run in that piece of crap... To make it short almost anything from autocad design and 3d max modeling to light image manipulation to light video edition, reproduction of high resolution video playback (no HD 1080p since when I upgraded to my current mac they did not existed or they were too expensive) but i did saw a few 720i movies on that computer (with the original sound card, the sound and audio would get desynchronized every few mins, but once I "upgraded" it to a $15 generic one, everything was back to fine again) I could also play several "modern" games at mid-low resolution (usually 1024x784 since the display was 15",) I think the last game I played there was a Need For Speed ( version either 7 or 8, I dont remember since there are so many! and I havent played at all in 2-3 years due to school.) The whole system would boot up Windows XP SP2 (yes!! XP) from the time you pressed the power button, to the time the system was ready to open in 45s (when no having programs automatically starting up with the machine) to 1min 30s with yahoo, norton 2003, and a couple of other programs (about 2-3 more, but not as intensive like norton or yahoo messenger.) I want to stress out, that the computer I had after that one and before my current MBP was a athlon X2 4200 @ 2.2Ghz 3gb RAM and 10,000rpm WD raptor, and windows XP (the same installation disk and everything) would load in 1min without startup programs (it took a bit longer to pass the bios screen, that probably caused most of the difference) and 1:20 with all the startup programs (this time was yahoo! and norton 2005 not 2003, and two more programs.)
My point is that although modern computers beat 1000000 times older ones in benchmarks, usually when it comes to actually running the program the gap closes, I have never seen a computer that claims to be twice as fast as an older one actually running a program in HALF the time, usually is seconds if not milliseconds.
Someone pointed out that the MBA is "full of bottlenecks" let me tell you a little secret, it can only exist ONE bottle neck which is usually caused by the slower component (traditionally the HDD, but now we got SSD so that might change from system to system.)
I do however agree that the Mac Book Aluminum is a better deal for your money, battery last more, you get better performance overall, and is just 1 pound heavier (I particularly would not like a computer such as the Air that is too thin and too light, since I'd feel I might break it by just typing, but thats me) also you get a NEW one instead of a refurbished for the same price. SO... if you really appreciate 1 less pound (notice Im using the weight to compare them, because if I use the word portability they would be even since its true the Air is lighter but the Macbook's battery last more, and thats portability also) and you wont be doing professional video/image edition etc go for the air, otherwise the MacBook is way better value for money.