I demo'd the RMBP in-store today, and I have to say I'm disappointed with its performance. When using the Intel HD4000 gpu, the interface has some noticeable lag during animations (mission control, enter/exiting fullscreen mode, launchpad, etc). It gets worse when you increase the screen real estate via scaling options. So, obviously the integrated GPU cannot handle the retina resolution, which means you need to be using the discrete nvidia gpu for smooth performance (and that, unfortunately, kills battery life). So, I gave the new base model MBA a try, and everything seemed great. My only concern, however, is I will be hooked up to an external 1920x1080 monitor most of the time and I'm worried if that will cause any UI lag like I witnessed on the RMBP. 1080p is a far cry from the 2880x1880 the HD4000 was trying to drive on the RMBP, but I figured I'd ask anyway to be sure.