I purchased my OnePlusX directly from OnePlus. They had a deal on Black Friday where I didn't need an invite. Ironically, I got my invite the day my phone arrived in the mail. I believe, right now, December 8, 2015, they are selling the OnePlusX without the need for an invite. Don't buy the phone here. It is $250 from OnePlus.net, which is the only official way to buy the phone.
On to the phone... I bought this phone as an upgrade from my well-used Google Nexus 5 (not the new 5, the original). One feature I really wanted was the ability to use an MicroSD card for memory expansion. My Nexus 5 only had 32GB of RAM, which is not a lot if you are recording video. The OnePlusX only has 16GB internally but I ordered a SanDisk Ultra 128GB microSDXC
at the same time I ordered the phone. The phone arrived form China well before my memory card came from Amazon even with Prime (I think it was due to holiday shipping delays).
The OnePlusX is reminiscent of my old Google Nexus 4 with the glass back. The OnePlusX comes with a rubber case. I also ordered the slate-style case. The phone is almost exactly the same dimensions of the Google Nexus 5 just a little thinner (even with the case) and a little more squared off at the corners. The OnePlusX feels more substantial than the Nexus 5 and is only a fraction of an ounce heavier. In terms of form-factor, the OnePlusX is a great replacement for the Nexus 5.
The screen on the OnePlusX (amoled) looks amazing compared to the Nexus 5 (ips). Both phones are 1920x1080 at 5.0 inches diagonal but the amoled screen looks great! Both phones claim to use GorillaGlass 3 but the OnePlusX feels stronger. One of my complaints about the Nexus 5 is that the phone is a little flexible. The OnePlusX is very solid.
The system is a lot more responsive than the Nexus 5 even though my Nexus 5 upgraded to Android 6 and the OnePlusX is still on Android 5.1. So far, the OnePlus extensions (OxygenOS) have been unobtrusive and somewhat handy. The gesture to turn on and off the flashlight when the phone is fully locked has come in handy already. The OnePlusX does have a slightly upgraded processor and 3GB of system RAM compared the the 2GB of system RAM on the Nexus 5.
I haven't used the camera yet but will try to update the review when I do.