Both my wife and I use Samsung Galaxy S5's and they seem to switch to whichever 5 ghz is strongest, so I guess in that sense, they "roam". Obviously our two laptops do not, but I mostly use mine upstairs in the bedroom (in which case I'm on the "ASUS 5G" upstairs). I use an Asus USB-AC53 adapter in my HP laptop, which only has a really crappy Ralink wireless b/g/n nic...with the AC53 I can connect to the 5ghz channel with AC at between 830and 720 mbps...which in the real world gets me around like 50+ on large file transfers over the network, and at least 250+ download from the internet). I'm using the laptop downstairs in the den (which is where the repeater is located), I just switch to "ASUS 5G2d" depending on what I'm doing, e.g., if I need more speed. But for the most part with my laptop, I could stay connected to the upstairs router if need be since I get decent coverage almost everywhere in the house, just not top speeds at the fringes of the router's coverage and thus I switch it manually when I'm downstairs..Not a big deal. My wife's laptop is usually in the bedroom, and so she stays logged on to the upstairs router; her desktop is located in a back bedroom downstairs, and it's got an AC dual band wireless NIC pre-installed, so it connects wirelessly to the repeater, as does an HP Deskjet and a SmartTV located in her office. Upstairs I have several FreeNas servers, another HP Deskjet, a Win8 Server, and an old XP machine, all wired to the router with Gigabit Ethernet (and with a separate Gigabit switch), and another SmartTV. In another downstairs bedroom we have a Wii, a Slingbox (one of our sons is in Toronto, the other is in Israel and they both use it regularly to get programming they can't see where they are, at least not live) which connects to the upstairs router (the bedroom is located directly downstairs from the router so it's closer than the repeater) using a Netgear WNCE2001 media adapter (which I'll be replacing shortly with something faster and more reliable), and another desktop connected wirelessly (using another AC USB dongle). All of this stuff connects and works flawlessly, and every device and computer on the network can access the shares (the FreeNASes and Win8 Server) for music, files, and movies. No trouble streaming 1080p and even Blu-Ray rips.
Both the 5ghz router and repeater need to be on the same channel, and after using Inssider and several other tools, I find that 161 is the cleanest and best channel to be on since there are no other conflicting routers anywhere on our block that use that channel. And since at the moment there's no one in our area using anything in the upper bands, I'm using the 80mhz setting for the channel width (yeah, it would be anti-social if there was anyone else on those upper bands, but there isn't so I don't worry about it). On the 2.4 ghz channel, we have a neighbor who idiotically set his router to channel 7, so he overlaps with channel 6 (which seems to be the busiest in our neighborhood), but thankfully we're the only ones using channels 1 and 11. So all is good.
Our two sons are grown and no longer living at home, but when they visit, one is an Apple-holic (multiple i-phones, multiple macs for business and personal), and the other is an Android/Windows/Linux guy, and they never have any issues getting great coverage anywhere in the house. And of course, the AC66's (and the 68's and 87's) have the ability to set up at least two different "Guest" networks with secured logins, and I often set that up when we know we're having a lot of people over for dinner, drinks or social events.
But for the most part, the majority of our devices other than laptops, phones and tablets are pretty much stationary (e.g., desktops, printers, smart TV's, HTPC, Blu-Ray and HD-DVD players, Wii, X-Box360, Roku, Chromecast, Fire TV, etc.) and they don't need to automatically switch between the router and the repeater. We have several tablets (including a Windows Surface that I love) and they do need to be manually switched since they don't detect which signal is strongest, and we do that from time to time, depending where we are using them in the house.
But I find this router-repeater set up to be very workable, and it provides really solid and stable coverage everywhere on our property. It's a very flexible set up and best of all, it didn't cost too much to get it up and running.