Looks like a Samsung S6 with a USB-C port, running QCOMM 820 and supports 24 LTE bands (which's ~100% of all global carriers). Which is not a bad deal.
Overall I was fairly impressed with Xiaomi's new flagship device. The screen and design are definitely the highlights of the device. The Snapdragon 820 seems reasonable and represents a large improvement of Qualcomm's previous generation of SoCs. While there's still a lot more to test for the upcoming full review, in the short time I had with the device I didn't find any outstanding faults with the Mi 5 so it looks like Xiaomi was able to pull off a quite outstanding device that seems to be hard to beat in terms of price/performance ratio. Look forward to the full review in the upcoming weeks.
AnandTech seems impressed too.
NB: While QCOMM 820 seems to be a generational update compared to 810 and 808, I want to point out that part of the reason for it is that QCOMM messed it up with 810 and 808. The true comparison will be against the A10/A10X from Apple.
Oh, and it also has hardware fixed function VP9 decode up to 2160p @ 60fps (warning: pdf). And that's exciting.