I play with UOKR most of the time, and for me it's as smooth as a baby's butt. But it still have some CTD bugs, which can make it a bit too unreliable when you go into really dangerous territory, or do PvP.
For that reason i also have the 2D client installed, that i'm going to use for guild hunts and such. Problem with the 2D client is that it's max resolution is 1024x768, with a max gaming screen of 800x600, which becomes tiny on my 24' screen. And i cannot play in fullscreen mode since the 2D client does'nt support widescreen, so everything becomes stretched on the width.
But when i am soloing, and minding my own bussiness, i use UOKR about 95% of the time, and love it. It still have that UO feel, though with a small facelift gfx wise. Many complain about the change of controls, but UOKR can easily be changed to legacy control mode by pressing a button in the settings screen.
Hope this helps