I have a 2009 mbp with a SSD that I use for heavy renderings in autocad. I do it via logmein, so I ca. Do it everywhere. I download the files onto the mbp and then use it's power to do the heavier stuff. But 98 out of a 100 things my MBA 13 ultimate does fine so far. I only had it for 4 days now, but it have been heavy work days. And you'll notice it almost right away when it is underpowered for your daily work. But so far it has been a joy to haul around, thin, super tapered design for typing (really the most comfy typing since I started using laptops, and it's not only the taper, the keyboard is silk smooth and a joy to use), really good batterylife (or perhaps my mbp was ready for a fresh battery)and most of all it's almost half the weight.
With the amount of time I spend with my laptop on my shoulder in a bag, I'm not going back, I'll keep it light weight!