Not really a full review, but some initial reactions. UPS brought me my new HP x3 yesterday. It took me several hours to get it set up with all my apps and the most recent release of the anniversary update (14393.189), but it is squared away and running. I am for the most part very pleased with it, though I won't retire my 950XL to use this one exclusively. In some ways it is either better or more appealing than the Lumia phone, but in other ways not. I didn't expect the camera to be as sharp, and it isn't. That may improve with future firmware and imaging software updates, but if it doesn't, I still have the better cameras in other devices to back it up. I never travel with just one phone in my pocket.
The phone's specs are fine: 4G RAM, 64G storage, 820 SOC. I ran the Antutu v6 benchmark on it and got 109189; the same test suite on my 950XL gave ratings between 83000 and 84000. My 6S+ still tops all with 128842, but Apple is famous for squeezing top performance out of upper mid-range to slightly high-range processors. Strangely, the fast and powerful x3 seems briefly draggy from time to time. Subjectively, the unit is slow to light up with a fingertip wake. (I don't use iris scans, which strike me as inherently slow anyway.) The fingerprint unlock is smoother on both my 6S+ and Nexus 6P. Still it works more quickly than the 950XL experience of sliding up, waiting for the PIN pad and tapping in the code.
The screen is bright and sharp, which is to say no worse than than the one on the 950 XL and maybe slightly better.
I prefer the feature layout and design of the x3 to that of the 950XL. The power button is on the upper half of the right side and both volume switches below. Locating the 950XL power button between the up and down volume buttons was simply a mistake that should never have been made. Since I am no longer worried about sealed units that won't let you replace a battery, the unitary case design of the HP looks like an improvement. In years past I used to insist on carrying an extra battery for a phone, but after a trouble-free year with the sealed 6P and 6S+, as well as the openable 950XL for which replacement batteries could not even be purchased without ordering abroad, I have realized that openable cases are simply not necessary. It's also good to have a sealed case because that forces the SIM/SD slots to be accessible more conveniently on the side of the case rather than being buried deep inside. There is enhanced water protection from this design as well. The x3 is rated IP67, which is not technically "waterproof," but should allow you up to half an hour to recover your phone from water less than a meter deep.
This isn't intended as a review, so I won't run on. My overall impression is favorable, and I will not be returning it. There is a lot here I want to explore and, if possible, exploit. There are a few design decisions I don't like (the forced choice between a SD card and a second SIM, for example), but in the grand scheme of things there are easy workarounds for minor limitations like that. If I run into serious limitations and disappointments, I will come back and report them.
I will acknowledge the hey-look-at-me packaging without judging the phone on the basis of its container. The enormous box is serious overkill for a phone-and-dock combo, but adjustable interior partitions will probably permit the eventual Lap Dock to be shipped along with the phone and even other accessories as the market for the x3 system matures.