I'm a long time FF800 user. Recently I decided to get UFX to replace my two FF800 units. I have to say it was a good decision! UFX is full of new features I did not miss but now I would not want to live without.
Two of the four mic pres no more share line inputs which gives me two more channels. I got used to 56 inputs with two FF800 but now I try to survive with only 30.

How about sound quality? I'm pretty sure UFX D/A converters are provide more natural and clear sound than FF800. Sound difference is not "day and night" but very subtle, even so subtle that I may imagine it: "I have a new, expensive piece of hardware - it must be better than the old one". How about A/D? I don't know yet... Yesterday I recorded & mixed a band using line and ADAT (Apogee AD16x) inputs but not preamps. Line input channels sound quality was second to none. For now on, audio interface converters will be the last weakest link in my studio.

I use SONAR X1 as my main DAW. I tried buffers from 128 to 48 samples and they all worked perfectly. 48 may be too tough in large projects but it seemed to work well with 30-40 active audio tracks and plenty of FX.
There were some install issues, though. My system did not recognize UFX when connected via USB. After installing driver manually it still did not work. I tried to connect UFX to different (ASUS P6T mobo) USB ports  but it did not cure the problem.  So I connected UFX via 1394a. Firewire works almost perfectly but UFX has disconnected from the system for a couple of times which means suddenly no program can not initialize audio playback or recording. It happens equally when connecting UFX to the mobo's Firewire 400 connector or to TI based FW 400 & 800 card. Thus it should not be a chipset or FW interface problem but something else. It's not UFX problem either because I had similar problems with FF800.

Totalmix: new features are more than "nice to have". Per track EQ and dynamics are a great addition and built-in reverb is useful in tracking phase if singer does not like the dry sound.

Thanks RME, you have done a great product once again!

-Panu