A little about my phone history/usage - I've had £100-£150 smartphones, Samsung S3 mini for the last 2-3 years so that's what I'm comparing against. In terms of usage, I'm on EEs lowest monthly plan and nowhere near my phone/text/data limits (phone/text might be unlimited, can't remember). I use my phone quite a bit, as an MP3 player for 1-2hrs on a workday, less at weekends; browse urban on tapatalk for 30m-1hr most mornings, occasional phone/text messages and as my camera. Maps is rare but critical and googling stuff is reasonably frequent. I'm mostly around places with wifi.
One thing you need to know for the purposes of this review is that the program that puts the standard android display onto the eink screen is called yotamirror.
I've charged the battery on:
Fri/Sat night, Sun morning, mon/tues night, tues/wed night, fri afternoon, sun morning. First bit of the week I was absent mindedly using the LED screen, from Wed I've tried to just use the eink screen. Seems to roughly double the batter life, so 2-3 days, rather than 1-1.5 using the led screen. So that's a big difference but also not revolutionary as I'm almost always somewhere I can charge each night anyway, but if you're struggling to get through a day then it'll make a big difference to you. I'm sure it'll make a big difference to me when I'm on a train and I've forgotten my charger or if I find myself out and about all day needing internet. Battery takes about 2 hrs to fully charge from flat.
I've found that the eink screen needs larger font size, possibly because of the ghosting (which I'll come on to), or maybe just the nature of an eink screen. I've set the accessibility setting to large which set the font to huge and maybe some other things and it's much better, I've also set the android home screen to a plain black background which looks weird on the led screen but much better than a patterned background on the eink side. I also prefer white text on black background.
What I've been using the eink screen for
Urban/tapatalk, texts (with the android app not the native eink app), gmail, notifications, clock
What I've been using LED screen for but can do on the eink screen
Phone calls (because it's weird to make/recieve a call and need to flip the phone over for the mic/speaker).
Camera and maps because the resolution/colour makes a huge difference and also with the camera you have to use the front camera with the eink screen which means low res and no flash.
anytime I can't see something I think should be there when I'm using yotamirror
anytime I'm in low light
What I've found I can't use the eink screen for
nothing. I've used maps and the camera on the eink screen, I've watched videos and looked at photos, and it's useable, especially maps. Some photos/videos don't work (edit: I mean they display/play but can't really make them out cos b/w + low res) but I can take a picture just fine only it's with the front camera so lower resolution and no flash. I won't ever use the eink screen for them (unless I'm running low on battery), but I can, just it's much better on the led screen due to colour/resolution/better camera.
Downsides
There's no denying that the led screen offers a better experience in most ways - better touchscreen, higher resolution, colour. Some of the problems with the eink screen would be resolved if it was the primary or only screen such as with phone calls. It's annoying to have to move in and out of yotamirror, just because it adds extra steps and you have to unlock the phone twice (once for the eink screen and then again when yotamirror puts the android screen up, I'm assuming it is using the cast screen function to do this). I'd prefer it if the eink side just had an always on lock screen and then unlocked directly to the android screen.
In theory it'd be better to have all eink native apps but the reality is that the apps/widgets that are on the phone are mostly not as good as the android ones - phone, clock, date, battery life are all good, but the text message app (and anything else) won't use swype which kills it for me, I find swyping so much quicker and easier. The music player widget is shit, doesn't click through the the mp3 app (I thought android had a native one but couldn't find it so donwloaded one from the store) and although once you've got an album selected you can play/pause and fwd/rwd track, sometimes it loses this and if you press play without anything selected it just plays shuffle. MP3 player is, in terms of time, the most used function on my phone so this is really important to me. I have no interest in the other eink apps like the fitness app or the games (2048, solitaire, something else).
The ghosting on the eink screen, it doesn't refresh that often, always at regular points, usually when you hit back but not always. So with urban in tapatalk it only refreshes when you move in and out of urban, not when you go in to read a thread, or go back to unread posts/subforum. So when you're scrolling down threads, it leaves a ghost of what was there before, nothing like screenburn, because the text overlays it completely so it becomes a patterned rather than flat black background which is not as nice to read off. For other people this effect might make the eink screen unreadable in certain programs, which would also include general internet use - anywhere that you are scrolling. I'd try to get some photos of this but the only camera I have now is on my phone

I'll put some photos up when I can borrow a camera, or rig up an elaborate set of mirrors
The touchscreen has a lag - I'm assuming this isn't actually the touchscreen but down to the lower refresh rate on the eink screen - friend says it's better than the kindle touchscreen though. It needs getting used to. It's not disastrous at all.
The phone itself is a little too big, and I haven't had the "bumper" (protective case) yet which'll make it wider. Maybe I just need to find a different way to hold it but it's slightly to large to hold and use in one hand for any period of time. It only needs a few mm less width and it'd be fine. There's also a huge bezel at the bottom, which may be needed for the bumper to grip on of course.
I miss physical home/back/overview buttons, I think they've not got them here because they need those buttons to show on the eink screen when you're in yotamirror.
e2a: also no external sd card slot which I'd really like. Not being able to swap the battery not so much of an issue with the increased battery life and the amount of external battery packs you can get these days.
Upsides
Battery life is a big improvement.
I prefer reading eink rather than led and most of my phone usage is text or voice so the eink screen works really well for me. If you're more visual, photos video etc then I don't think this'll work, certainly an eink only screen phone wouldn't work for you.
All the apps/functions/widgets I use work perfectly well on the eink screen, there's nothing that I couldn't use, although I'll use the led side for maps and camera.
The always nature of the eink display is fantastic - I can just glance at it to see the time (my phone is my watch) and can do things like lock it on the kitchen timer app when I'm cooking and it updates so I can see the timers without having to touch my phone. I've got it set to show yotamirror for 20minutes before reverting to the eink lock screen.
The phone itself is light and slimmer than my s3 mini despite the extra screenage. It feels well built, high quality. The led screen is fantastic, really bright vivid colours, great resolution, superb touchscreen, much better than the S3 mini. Larger screen is good too but as said, bit too large for my hands.
Overall
From that it seems like the downsides are bigger than the upsides but that's not right. If the eink screen was the primary or only screen and went straight to the standard android screen, the first two paragraphs of the downsides would be gone. The phone could be smaller (something in between the S3 mini and this would be perfect for me). The only downsides which can't be sorted are down to the nature of eink vs led screen - black/white display, lower resolution, lower refresh rates and the ghosting - so it depends how much any of these matter to you.
Also, the downsides are all pretty minor really (aside from the lack of colour) whereas the upsides - the battery life and eink display - are huge. If eink screens take off then we could see more apps being developed for eink screens which would improve things but I think I'd still prefer the phone go directly to the standard android stuff with all apps running from there, and just a different lock screen because of the always on factor.
So I'm very happy. When this phone dies/gets broken I would go for an eink only phone if it was significantly cheaper, or a dual display phone again preferably with the eink screen being the larger/primary screen. Although more likely I won't be able to afford a phone like this again and I'll be back to £100-£150 smartphones.