I was used to a Windows 7 desktop, have had 16Gb RAM and 512Gb SSD in the last two desktops so the rMBP is not quick by comparison. I only bought the MBPr because of the 768Gb option (on a desktop, 512Gb is more than enough as a main drive as you can use a spinny disk as well). Win7 apps in general seem to be significantly faster than their OSX cousins, presumably because most of them are written for Win7 and ported over. Or perhaps for some other reason. Video conversion, for example, takes minutes on the rMBP but seconds on a PC with similar specs. iMovie is a joke, spending an hour importing a video before keeling over. If you're used to video processing on a PC it will take a while to get used to a Mac. Similarly, the general wobbliness of OSX is a bit of a distraction and the number of times I have to reboot (or attempt to reboot, but the spinny ball won't let me, or some other app complains about rebooting) is a bit of a backward step compared with Win7. But not the end of the world. The one thing that completely infuriates me is the Finder, and its complete lack of any form of apparent design by someone who's used a computer (or looked for a file). But I understand that most Mac users feel the same, and we're stuck with that for the moment. There are plenty of nice features -- spotlight, and the integration with iOS devices, to name just two.
TL;DR. It was very much easier than I thought to move from a serious Windows setup to a rMBP, even though I have plenty of critical applications and can't have them down for any length of time. I don't use Windows (virtually) nearly as much as I thought I would; I don't use Bootcamp at all really.
I'm really happy with my rMBP. The only concern I have is what to do if it dies and throws away my data, which is more likely than with a PC (it's already done it once on the upgrade to ML, which Windows has never done to me) and much less easy to fix (on a PC you just chuck the drive into another PC). For that reason I religiously back up with Time Machine (which is what saved me after the ML upgrade), but I am also looking at storing a fair bit of stuff on an SD card, which I could then use on a MBA or similar if I convince the missus to get one.
Here in good old NZ, we have no Apple stores and so if the rMBP dies I am completely stuffed. So I'm a bit living on the edge without a spare Mac I can potentially transfer to.