I bought the retina deal last month on FNB.
Loving mine, although its quite a heavy machine to carry about with you.
I needed to get a larger bag to carry mine, ordered a nice leather one on Amazon that I'm still waiting on (its here, but I need to go to my friends to collect it, as he brought it back for me).
No speed issues, and the screen resolution increase is great.
I usually run mine at Scaled / "More Space" in Displays so that I get to use the resolution increase.
Video seems ok - I tested out my latest 4k tv with it yesterday (bought a 49" yesterday), and have an older 39", and could play 4k video without any hitches. Still have to try h.265 and see if the cpu can handle it at that resolution, but h.264 is fine. Mouse was a little jerky on the larger 4k screen, but thats a function of HDMI1.4 being stuck at 30fps.
None of my 4k tv's have hdmi 2.0 or dp connectors
256G is a little limiting, but not overly. I usually keep all my media on the venerable HP N54L NAS's, so its only VM's and dev work stored locally.
You can force VMWare Fusion to use the actual resolution for Windows, which means I can have dinky sized VM's for crappy bank apps that only use ActiveX controls (for your safety nogal!), or other crappy windows software like the rather crashy config software for my solar inverters.
Luckily for me, most of my dev stuff can be done natively now, I use homebrew quite extensively - brew install XXX...
I also only have the odd windows only stuff to deal with every now and then, so I don't have to keep windows around except for rare cases, which is nice.
I'll probably upgrade the drive to a larger unit at some point. I have a 512G floating around that I can use that a client gave me when I upgraded his Retina to 1TB (more money than sense, given the cost, but hey, free drive!).
I usually stick the old drives into HDD cases and reuse.
Apple keeps changing the sockets on the drives which is a bit of a pain, but I can get any of the adaptors for them relatively easily. Currently its a not quite standard PCI-E socket for the SSD, which I have USB3 and SATA adaptors for. Drive is removable and upgradable (I can get up to 1TB, but its $$$$$), RAM is not upgradable.
16G ram is plenty fine for most apps and VM's though.
If you look around online you can get a cheapish case - ZAStore (of all places!!!) had 15" retina ones in matt black for R200 odd.
All in all, its a great great machine. I still need to buy Applecare for mine, and put it on my insurance. Buy AppleCare.
My previous FNB Air was crap - multiple failed SSD's, motherboards, then finally *they* dropped the machine and dented it!, Apple just replaced it for me under AppleCare with a new machine last month (one of the reasons I bought the Retina, I couldn't wait for them to replace the Air). Took them 2 1/2 months to replace it.
FNB's insurance for the unit isn't a great deal, my household insurance is cheaper, so doing that.
May be different for you though.
Lastly - it doesn't have Ethernet.
That may be a big caveat. You can use the thunderbolt ethernet adaptor, but its a ton flakier than having a socket.
If I do any router firmware stuff thats time dependant on tftp'ing files over at router boot time, I usually have to pull out another laptop with ethernet. Thats about the only downside really (for me).
Battery is good, but not great great like the current Air.
I get about 5hrs good use out of the machine, which means I can almost use it for a days work before I need to get to the power again. If you have older Mac's about at work, buy the charger adaptor for it too, so you can use their chargers. I often don't have to bring my charger as I can use someone elses for a short period at clients

Your mileage may vary though!