I have it, being using it since the official launch in UK. It's the best dual sim smartphone out there, full stop.
It has everything that is good about the original HTC One, speed, build quality, appearance, great camera, good 3rd party compatibility, Sense UI etc. on top of that, the external SD card gives the phone storage up to 96GB: 32GB onboard + optional external 64GB (i m using it with Sandisk Class 10). As it is an official UK Model, it comes with all the standard google apps integration, basically ready to use out of box (there is no need to fiddle with root or ROM). It's also due to be upgraded to KitKat mid-end Feb.
In terms of dual sim capability, it just works. the 2 sim cards are simultaneously active in the truest fashion (i.e.you can call line 1 with line 2 or vice versa). The build in apps have all been optimised for the dual sim capability. Also, the battery life is outstanding, comparable to the single sim version.
The only drawback (if any), is there is not 4G LTE support for this. Also, unlike Samsung DUOS series, there is no option to select which sim is used for 3G and which sim is used for GSM. the default Sim1 is WCDMA, Sim 2 is GSM. But I guess it's more of a preference, as the fixed setup makes the setting simpler and straight forward vs Samsung DUOS. I typically had to restart the phone several times on DUOS to get the correct 3G works, but on the HTC One Dual Sim its straight forward: Sim 1 WCDMA, sim 2 GSM.
I think this is a no brainer, for anyone that's going to get either a Dual Sim or a HTC One, this is the version to go for. It's better than the standard HTC One in every single way except the lack of 4G LTE. And, its simply the best dual sim phone out there by miles. It's slightly more expensive (