I am running Win 8.1 + UR44 - it is flawless. Cubase reported latency is at best 4.x input and 5.X output. I really don’t know what that means because even when I change to higher buffer settings – asio guard provides some magical low latency for VSTs where it feels like zero latency.
The on board reverb is beautiful ( I had a Focusrite Saffire Pro 24 where the effects were terrible)
The integration with Cubase is what sold me - No external s/w mixer required.
FYI - I am using it on a fast win desktop and have also used it on my iMac combined with a Roland quad-capture using the OS X aggregate device . Worked perfectly.