HTC's new flagship U Ultra hasn't been out that long at all, and is still awaiting release in some areas. In spite of this, it seems there is already a port of TeamWin Recovery Project, or TWRP, available for download at XDA Developers courtesy of Captain_Throwback. The unofficial port is of the latest version of TWRP, built directly from the latest upstream sources, and works as expected, with theming, flashing, booting into the bootloader, and wiping the host device all working as expected. The only wrinkle with this version of TWRP right now is that the option to reboot to recovery from the recovery menu doesn't work, meaning that anything that needs a recovery reboot before or after flashing is a no go. According to Captain_Throwback, getting this feature to work correctly will require the kernel source for the device, which HTC has yet to release.
Installation happens via the standard Fastboot method, which requires a PC with the proper drivers installed. Like most recent HTC devices, the U Ultra ships with an unlockable bootloader, and even maintains warranty coverage if the bootloader is unlocked, though certain actions that an unlocked bootloader makes possible, such as flashing custom ROMs, can void the warranty. The HTC U Ultra has reached a few areas so far, but major markets like the United States and India don't have the device as of yet, which makes it none too surprising that those who currently have it and decide to flash TWRP won't have much to flash at the moment - in the way of things like custom ROMs, kernels, and radios.
As a recap, the HTC U Ultra is one of HTC's first "glass sandwich" devices, bucking the company's established design tropes in favor of contoured back glass and a secondary ticker display that apes LG's V10 and V20. The internals feature highlights such as the same Qualcomm Snapdragon 821 processor found in Google's Pixel devices, alongside 4 gigabytes of RAM and 64 gigabytes of internal storage. The 5.7-inch main display features a Super LCD 5 panel with a resolution of 2560 x 1440 pixels and a curved sheet of Corning Gorilla Glass 5 on top. The device can currently be pre-ordered unlocked, directly from HTC, for $749. The front of the device is black on all variants, but the back glass can be blue, black, or white.