I bought a full RME rig made of a Fireface UC, a Quadmic and an ARC controller. Overall, I'm happy with the quality and general implementation of the RME products.
However, of the three pieces I bought, the ARC Usb is the one I'm the least happy about. The controller has in fact some arbitrary limitations in regards to MIDI configuration and programming.
More specifically, I wrote a custom script for the ARC which was supposed to make it behave like a mixer controller for the RME UC (within its own TotalMix software) and as a generic MIDI controller within my DAW, using the ARC buttons left unassigned on the mixer. After inquiring to the RME people on their forum, I now understand that they do not allow the ARC to be freely programmed, and that the controller will only work within the audio interface functions.
I'm extremely careful in keeping a minimalist working environment. I spend a lot of time optimizing the usage of my gear, and making sure nothing sits on my desk under-utilized.
I now have 8 unused ARC MIDI buttons sitting right by my left hand, which could have been easily used to control basic functions within my DAW (i.e. play, rec, stop, tap, metronome, and so on), saving me space and money for another controller.
I really did not expect this from RME. If I buy a pro-grade product, I expect to be able to do some very deep re-configuration. It's the product that must adapt to my workflow, and not the other way around.