In 2012, when I decided to switch... spurred on by a horrible HP experience, my original plan was to get a iMac + MBA. As has already been stated this seemed to be the best option in terms of processing power and screen real estate in my home office, and portability when on the road.
But, because I wanted a new iMac spec, I decided to buy a rMBP. It's an amazing and powerful machine, but (for me) there has always been too many trade offs when I am in the office. I don't work in multi-media, but I do need a few applications open, 27" of screen real estate is perfect, I am a casual (but graphic card intensive) gamer, and the extra grunt is handy.
Given all of this, I have ordered a brand new iMac and it is traveling to me now.
I will keep my rMBP. It's a bit heavier than I'd choose for 'on the road' and is a bit of overkill once I have the iMac, but hey, if something already 'just works' why change?
You'll be happy with a rMBP and, depending on your needs, it might be sufficient, but if circumstances allowed, I'd been better served (and financially better off) with an iMac + MBA.