So I was at my local Apple store earlier today comparing the new 13" and 15" rMBP machines. They had just received them and were setting them up. I was the first to take them for a spin. I am a desktop and mobile application programmer who does quite a bit of gaming to give you an idea of what kind of things I expect from my machines portable or not. I figured some of you might want another hands-on impression of the machines. When I say that I am maxing a system's resources or I am taxing the system it means I am doing the following: I loaded all the thread pipelines, maxed the RAM to the point of paging to SSD, and was writing a big file to disk all at the same time. I came away with the following impressions: The screens on the 15" machines are a step up from the 13" machines in terms of clarity and brightness. The Iris Pro makes a noticeable difference compared to the Iris in the fluidity of the UI across the system both when the system is being taxed and when it is completely at idle. Scrolling the iTunes library and App Store is most often quite smooth but occasionally jumpy/jittery on the rMBP 13". On the base 15" rMBP it is almost always (read 99%) buttery smooth. That 128MB cache really shines here. The performance increase that comes with the 100Mhz difference in clock in the Iris Pro is minor\negligible relative to the performance increase the 128MB cache provides I believe. The 13" rMBP with its resources maxed out is shockingly quiet. The 15" rMBP with its resources maxed out has quite a bit more audible fan noise than the 13" rMBP but still very respectable. The size and shape of the new 13" rMBP has made the size difference with the 13" MBA negligible in my eyes. It only comes down to whether you value screen and processing power or battery life. 15 minutes on the new retina screens will ruin the non-retina screen machines for you. I didn't think much of retina at first but now I don't want anything else. Take my impressions for what they are worth and with a grain of salt of course. I hope this post will be of some benefit to some of you.