I use for work a similar Skylake-U processor based laptop and, since it works nice, I decided to switch from my old sandy bridge 2630QM based laptop to a Skylake-U.
So I got an Elitebook 840 G3, but with 512 GB M.2(SATA) SSD, 16GB RAM, another 500 GB 2.5 inch SSD(Samsung 850 EVO - put by me), LTE modem and fingerprint reader. Laptop works nice, very fast.
I am a bit disappointed about raw power as I realized my own personal laptop scenarios is more towards multi-core than fast single-core, so I see this laptop a bit slow when having background activities(Dropbox, Google Drive syncs, antivirus scanning, searching, copying...), as applications today tend to use more than one thread so you will see many times 100% processor usage, though opening one app is fast, browsing is fast....
After some benchmarks it seems the single core performance is very high while the others are just good, reason being the HP throttling to keep temperatures low (exaggerated if you ask me, on 100% usage laptop works at 55 degrees and it's cold...it could have went to 80 degrees and have more power).
See here
http://www.userbenchmark.com/UserRun/1987033 ; Single core score is similar to core of i7-6820HQ and over 6700HQ but of course, quad-core and multi-core (at least 8 threads) is smaller. SSDs, CPU and GPU over 90th percentile hence high performers for the models but memory not so good (probably because I use different models of DIMMs in dual channel).
I connected it to a 4K monitor at 60Hz through DisplayPort and works well in browsing + office but sometimes it feels the integrated graphics + cpu struggle (to watch a 4K 60Hz video on youtube you need to deactivate the laptop integrated monitor so that GPU has only the external one to handle), then usage is 100% of both CPU and GPU and the video is almost stutter-free.
The battery can keep between 2 hours and 10 hours. 2 hours in full load and 10 hours in very light usage. Most possible scenarios for me would be very light office work - 7 hours and software development with lots of browsing - 3 to 4 hours, which is OK considering the screen, multitude of components and weight(small 46Wh battery).
USB ports are fast. I connected an external SSD (max 320 MB/s in SATA) and it can reach 250 MB/s.
Audio is good, very loud through 3.5 jack, loud enough and very clear through the laptop speakers. Some issues were present at the beginning with drivers as it seems they were disappearing hence I had to reinstall a couple of times. Now everything is OK.
Screen is great (the QHD one). It has a very thin anti-glare layer which is more like semi-anti-glare. There is a bit of reflection compared to other brand anti-glare monitors, but it's also very nice as it's more near to glossy colors. By comparison, my 4K 27 inch monitor is of lower quality with a bit more grainy feeling. It needs a higher luminosity if you keep scaling at 100% (as I do), in order to differentiate characters, and this consumes more battery. The 210 ppi is great, retina-like.
As I said, very light, 1.535 kg with 2 RAM modules, 2 SSDs, WWAN, WLAN, QHD screen.
I recommend the laptop as it's a complete fast mobile work solution. If need to use it as a workstation, look at bigger models.