Finally got mine yesterday.
(again - maxed out refurb'd 27" late-2013; 3TB Fusion Drive, 4GB VRAM)
This one came with 8GB RAM, but I had already ordered 2 Crucial 16GB kits for upgrading the memory. Once I got it up-and-running and felt that the computer was working properly, I took a deep breath and installed the Crucial kits. Absolutely no problem. Whatsoever.
I'm running into some hiccups with getting Boot Camp and Win7 installed, but I don't think it will be a big deal.
I've also got a third-party Blu-ray writer running along with a WD My Passport and an Anker USB 3.0 hub.
My ONLY (very minor) gripe, is that I need the keyboard with the numeric keypad, and it's wired. Not sure why Apple can't produce a wireless keyboard with numeric keypad, but like I said - it's a minor complaint.