I have a 2012 27" iMac, a 2014 13" rMBP, and now the rMB. Granted I have only had this thing just shy of a week, however, installed just about everything I had in the rMBP and am using it as a daily driver as I had with the rMBP. I need more time, however, it has not hiccuped yet. Just this morning even, have 2 Powerpoint presentations open, Outlook, Parallels/Windows 8.1 to work on some Excel Files with VB/Macros, Safari looking up misc stuff, Feedly, Messages, Streaming Satellite radio connected via bluetooth headphones, bluetooth mouse and keyboard connected as well, background apps running : Dropbox, Creative Cloud, Google Drive, One Drive, Wunderlist, and WebEx ... a normal day

Lag is there but like I stated before, only in app switching and scrolling which my i7 rMBP exhibits as well.
I could see this as a main machine easily. Giving it another week or so and if all is still good will likely put the rMBP up for sale.
By the way ... unplugged the machine to see with all I have open what estimated battery life is and its showing 4:26.