From an hardware point of view, this is somewhat an evolution of the Nexus 5. Same size, same processor family (just a more modern variant), a little bit more of memory, no 802.11a, no NFC. The casing is quite beautiful and it comes with a good-looking USB cable and many protection layers (for the screen and for the back). It's a bit expensive considering it only matches the hardware of a Nexus 5. But maybe the AMOLED screen and the back in glass are more expensive. Also, great AMOLED screen.
And it's double SIM (dual standby, one radio but any SIM can receive calls and SMS when standby, only one sim can use 3G/4G, but it's possible to switch from one SIM to another; almost impact on battery). Good battery live (better than my previous Nexus 5 when it was new).
From a software point of view, this is quite different. The OnePlus One was running CyanogenMod and this is not the case for the OnePlus X. You get OxygenOS, a proprietary derivation of AOSP. It's almost pure AOSP. Most of the small differences are optional. So, good. The kernel has been released, so it's now possible to run CM13 (official build). However, this is still Android Lollipop. No date given for Marshmallow. The OnePlus 2 has a beta of Marshmallow, but not this one. There is no privacy guard either. I think that OnePlus doesn't have the resources to develop a proper AOSP derivative and the OnePlus X is doomed to be the last one to get anything (as it is not a "flagship" phone).
There is an annoying bug with no solution. When using two SIM, the lock screen can freeze 2 times out of 3. This means that you have to wait 5 seconds before being able to unlock your phone. If you spent all days unlocking your phone, this is quite irritating. Despite multiple reports, OnePlus doesn't really acknowledge the bugs. There were multiple updates and none of them corrected this problem.
Cannot say anything about the camera, some people complained about the quality.
About CM13, encryption is currently unsupported and no plan exists for that. Dunno what are the technical grounds for that. Also, noise cancellation is defective (didn't test myself). CM13 doesn't have the bug about the lock screen and the battery seems far better with it (with a "black theme", since it's an AMOLED screen).
In short, hardware OK, but OnePlus is unable to develop correctly the software part. CM13 support is still at its beginning (2 weeks). Don't think that the OPX is great because the OPO was great.