I took the plunge with the second gen MacBook Air after seeing fellow devs at WWDC haul them around as if they were nothing. I initially noticed the slower HDD and performance hit, but didn't miss it after the first week. The mobility was a huge boon. I was working for myself and often meeting clients or working from the coffee shop, bar, etc. I held on to it until the MacBook caught up in size/weight.
Source: me. I write iOS apps.