I have UR824. Also had experience with Focusrite Pro 10 I/O and PreSonus FireStudio. I prefer UR824 between the 3.
What I like from UR824:
Clean and smooth sound.
Stable driver and mixing panel, not like the PreSonus, the mixing panel is very unstable.
Very easy to use and to understand the mixing panel (dspMixFx), not like the Focusrite, which is very confusing for the first time.
Like the PreSonus, can be used as stand alone mixer without PC. UR824 memorizes the last setting, so the next time I turn in on without PC, it function as stand alone mixer.
Good build hardware, all the knobs have the same feel, not too loose or too hard. Some interface doesn't have the same feel for all the knobs, that make them feel cheap.
2 independent headphone outputs (even though they are not loud enough for high impedance headphones).
And of course, the excellent DSP, EQ and Compressor for 8 input channels, and Reverb.
But so far, I didn't managed to use the VST plugins, RevX, and the sweet morph channel strip, with Reaper. They are not shown in Reaper fx browser, even though I have added the Steinberg folder in the VST setting. Those plugins only work in Cubase. Not sure why, still troubleshooting.
One thing need improvement is the headphone output, the gain is too low for 600 ohm studio headphones. My DT880 is not loud enough on UR824. I have to use the low impedance headphones, like my Shure SRH840.
Hope it helps