I previously had a 15" MBP i7 4GB high res glossy which I changed to a refurb 17" i7 8GB anti-glare which I still own. I got my 13" MBA 2.13GHz 4GB 256 this week and so far I really like it.
So far I haven't done anything CPU intensive so I don't notice a difference, the flash memory seems to really hide any slowness and feels snappier than my 17" MBPs intel G2 SSD but that probably needs a secure erase to regain performance.
The battery was worse when using a 3G USB modem, using wifi without flash or using clicktoflash it feels like 1.5 hours less.
The screen size doesn't bother me, it's the resolution that bothers me, I wish it were higher but then mac os needs resolution independence and running a higher res would probably shorten battery life. The MBA screen also has this nice purple coating with helps with glare which is nice and I hope they add to all there glossy laptop screens.
But it's good the portability makes me want to take it everywhere and it doesn't feel like a compromise between portability and performance.