First impressions:The box is minimalist in design, with a small logo and title on the front and a large picture of the camera, and makes me think of a certain fruit oriented manufacturer. The box is 4" x 4" x 2 1/8" and the cover lifts off to reveal a tiny pearl white (in my case) nugget nestled within. It has a gorgeous, slightly satin finish and feels of much higher quality than most other action cams. Labeling and graphics are minimalist and add to the feeling of quality and sophistication (yes, I'm waxing slightly lyrical, but this camera has exceeded my expectations by a significant margin). It has a good heft and feel in the hand, weighing in at 3 1/4oz or 89g on my basic Ikea kitchen scales.Unlike many other GoPro wannabes, this camera comes with nothing in the way of frills or accessories. Clearly Yi believe that the inherent quality of the product means that they don't need to bundle a mounting harness for your pet dolphin or raccoon, or anything else for that matter - no waterproof casing, no handlebar mount, no selfie stick adapter. NOTHING, other than a six inch microUSB cable. Part of me wants to dock one star for that as even the GoPro units come with a housing and basic mounts, but you are saving a Benjamin and half over the black version, for similar or slightly better video quality (more on that later), so I'm going to let it ride for now.If you've never held an action cam before, the small size of such a unit will probably surprise you. These are unto Micro 4/3 cameras as those are to DSLRs, or possibly even more than that. This Yi 4K unit is a mere 2 1/2" x 1 5/8" x 1 1/8" including the lens bulge on the front. It features a standard 1/4" tripod mount socket on the bottom, a small speaker grille and dual microphone pin sized pickups either side of the power/shutter button. The battery and microSDXC ports are behind a well damped and well sealed door on the bottom, and the only other salient points are the 2 1/4" (diagonal) touchscreen on the rear, and the microUSB port on one side.To get started, you will need to peel the lens protection film off, insert the battery (do NOT try and peel the little plastic tab off the bottom of the battery, that is designed to make it easier to remove the battery at charging time - it is a REALLY snug fit), and press the power button on top. A short musical chime and a single LED flash on top confirms that the unit is powering up, and within two seconds you are greeted with the welcome screen. A moment of turning the camera this way and that will lead you to realize there are no control buttons - this unit has a touch screen for control of all settings and functions. A few taps on the screen will whisk you through the very brief "how to" and you will be left with the default "ready" screen.Insert a card, as I did, and you may get an error message telling you that there is a card error. You should probably power the camera off, BEFORE inserting a card, as I'm sure they probably tell you in the manual, but then who does that?By default, the unit is set to record 4K@30fps, with stabilization disabled (stabilization only available at 1080p and below). Unstabilized video looks pretty good, mainly because the camera is light and it isn't difficult to hold your hand fairly steady. Stabilization does make a noticeable difference, especially at higher resolutions, but you will probably be using more juice to do so (tests to follow). My blank 4GB card told me it could record a little over 7.5mins of 4k@30 video, so the likely 64GB card ceiling will offer around 2 hours recording time, which is probably more than the battery can handle.All in all, first impressions are extremely favorable, and should make this the best value 4K choice for most action cam aficionados.(Update 1)My camera came with v1.07 firmware. By going to the yitechnology dot com site, and heading to support I was able to find the firmware file to update it to v1.09. Firstly, check on the battery cover whether the tiny serial number starts z16v13L or z16v12L to select the right version. Then, simply connect the camera to your computer with the included cable, save the file as firmware.bin, directly to the root directory of the camera's card, disconnect the camera, and power camera on. You will be asked if you want to update firmware, and after confirming, the unit will take about thirty second to apply the update and then reboot.(Update 2 - 7.14.16)Puchased two 128GB cards from a certain local well known photographic store, but had very different results with each of them:PNY Elite 128GB MicroSDXC Card with Adapter -UHS-I, U1, up to 85MB/Sec (P-SDU128U185EL-GE)Inserted this card and powered up with camera set to 1080p@30fps. After a very brief SD card error message, was able to check settings and see it show up as 119.2GB of formatted space. This equates to 9hrs 8mins and 54secs of recording time of 1080p@30fps, or 4hrs 38mins and 57secs of 4K@30fps.HOWEVER - if powering on the camera in 4K@30fps, the camera will come up with an SD card error, and take 20 seconds to sort itself out. I did a brief test recording and there does not appear to be any frame dropout, so this card may cope with that frame rate (See below for more info).SanDisk Ultra 128GB microSDXC UHS-I Card with Adapter, Black, Standard Packaging (SDSQUNC-128G-GN6MA)Prior to testing, I would have put my money on the Sandisk card working right out of the box, and it being the PNY which needed fiddling with, but, surprisingly, the opposite was almost true. I have yet to be able to get the camera to recognize the above Sandisk at all. The PNY is recognized after a brief wait, but whilst it might be possibly to custom format the Sandisk in order for it to be recognized, out of the box the Yi 4K will not play nice with this card. (More info below).A note on 4K recording.One of the things I had looked for when picking an action cam was one which lived up to the specs it advertised. There are a lot of sub $100 4K cams around which purport to record 4k@30fps (well 29.97fps actually, but let's not get too geeky), but many of them will do that frame rate at 1080p and not at 4K. This one does appear to do so, possibly.... VLC (my media player of choice) shows the file properties as being the full resolution of 3840x2160 at 29.97fps, but that could just be what the camera has stamped on the title of the file. I will need to look a little closer at this as when I ask VLC to play the Yi's 4K video at full screen on my BenQ 4K display, it has black bars down the sides, and does not fill the screen. This bears further investigation over the coming week.>>> Further 128Gb card testing:Having sorted out a minor technical gremlin with my USB3 Card reader (it does help if, *ahem*, you plug it into a USB3 port), I was able to benchmark all three 128Gb cards I now have available. In addition to the two cards listed above, I now have the following:SanDisk Extreme PLUS Micro SDXC UHS-1 Card with Adapter (SDSQXWG-128G-GN6MA)This card was recognized immediately, and without an SD card error, the first time I inserted it in the Yi 4K. It is one of the approved cards recommended by Yi on their website, so it would appear that their testing has some substance, and if you want hassle free results, you should go with their list of tested and approved hardware. I have the Elite-X version of the PNY card on order (not on the approved list of cards, but more in line with the Sandisk Extreme Plus for speed), which should be here in a couple of weeks, and will update this review once I have it on hand.Benchmarks of the three cards are shown in the accompanying images, below, and whilst all three of them have sequential write speeds that are close to or above the 30MBytes per second necessary for high quality 4K recording, the Extreme Plus justifies its price by being almost three times that fast for twice the price of the other two cards.FOR THE MOMENT - If you want to avoid issues, refer to the list of approved cards at Yi's site.Update 3Have just received a Samsung 64GB PRO Class 10 Micro SDXC Card (MB-MG64EA/AM) (the previous generation to what is sold currently). This is a U3 speed card, and works in the camera without error. I also updated the camera to 1.0.12 firmware and have found that the PNY Elite card which was previously 'quirky' is now recognized without an error message when powering up! I have recorded 20 mins of 4K30 and 4K30 Ultra footage, without issue. The Sandisk Ultra card, however, is still not recognized and must now be assumed to be incompatible. Kudos to Yi Technology for their diligence in continued testing and development.<<<>>> Accessories:I have just finished reviewing a third party dual battery plus charger bundle:EforTek AZ16-1 Replacement Battery(2 Packs) and Dual USB Charger for AZ16-1 and Xiaomi YI New 4K Action Camera (YI-90003) from YI TechnologyOverall impressions are highly favorable after just a few days of usage. The price of the bundle is made worth it by the pair of batteries alone, if you can put up with the minor quirks of the charger.