As much as I like the progress Apple makes on their line-ups, you get the feeling that this isn't a compelling purchase.
I feel most compelled to buy from Apple after a new tech is integrated a year and they add the raw horsepower. So, that was the late 2013 MacBook Pro Retina, not the first model. For iMac, this model version would be "early adopter" friendly, but for practical purposes, wait for the next iteration in 2015 or beyond.
My major concern is that this first generation of Retina iMac ends up being like the iPad 3 - underpowered and replaced quickly. There is a good chance for that scenario to play out here given what we know about forthcoming Intel processors.
I usually do not jump on the "wait for rev 2" bandwagon, but in this case it is likely a good idea.