The loupe eye cup is symmetrical. To avoid light leaking in from the back you need to turn your head and look at an angle, or jam the cup into your nose until it hurts. And if you do that it still leaks light. Bad design, but you can live with it. Also, the diopter adjustment is very touchy. Hard to get it just right. And on a sunny day you cannot do without it.
I'm having a heck of a time getting the 4k files into the editor. I have given up on Premiere Pro 5.5 for now, and using Resolve. Playback stutters and the FPS counter jumps all over the place. even if I drop the playback resolution to 1/4. This may be caused by my computer, an older Windows 7 with only 12GB of RAM. Probably going to need an upgrade, including a 4k capable monitor.
HD works great. I have had limited time to play with it yet, but shot some of both resolutions in my back yard and took it to the beach today. Everything shot at default settings, haven't even read the manual yet so haven't played with the more advanced stuff, but so far I really like the image and the color of the clips I've looked at. I also just ordered a 64GB SD card for the HD side of things since I think I'll be doing mostly HD stuff for now until I can get my editing station upgraded. I'm supposed to get a year of Adobe CC premiere, but so far I have't found anything about that, so I'll fire off an email to B&H shortly.
In short; so far I really like the camera, no buyers remorse here.