I didn’t intend to buy it as a controller, but for the synths, sample player and sequencer inside. Basically it works like a standard MIDI controller, you can use 32 pads and 8 faders to control functions in Renoise, as it is described here:http://tutorials.renoise.com/wiki/MIDI_Mapping - the other buttons seem to be dedicated Circuit’s functions only. 1 extra fader sends to channel 16 (the filter knob).
I think it can only send and receive on channels 1,2, (10 and 16). And I did not try much with it - currently still using the computer keyboard. 10 and 16 seem to be useless, as 10 transmits only 4 notes (?) and 16 is used to control parameters in the Circuit’s synths.
So, you could probably use channel 1 (switch to synth 1) to enter the notes, and use the keys C0 to C9 in channel 2 (switch to synth 2) to use some of the other functions in Renoise (transport control, track muting etc.).By switching between channel 1 and 2 you can also “double” the fader knobs virtually from 8 to 16 (8 knobs transmitting data to either one or the other midi channel). The user interface is simple, but very well designed. Usually there is no menu diving in the circuit, and you can reach most functions by pressing only one button - also the switch from synth 1 to 2. Little disadvantage of the simplicity of the hardware interface: The rather powerful synths can only be patched via Midi - there is a good synth editor for it available for free.
The pads are velocity sensitive, so they also transmit the velocity value to Renoise, you can switch through the octaves. I think as a substitute for a small keyboard on the desk it is not so bad. I like the feel of the pads more than of my old launchpad. At least as a keyboard it knows scales. After you chose the scale, you cannot really type a wrong key anymore and it can also remember chords for you. I think for programming the melody part in Renoise it is more pleasant than the computer keyboard. But it seems to be difficult for editing effects, and it lacks support of all the good features in the Renoise editor, such as transpose, interpolate, etc . 
Here’s a little demonstration - in Renoise it works basically the same:https://ask.audio/articles/heres-how-to-use-novation-circuit-as-an-expressive-midi-controller
If you want to control the Circuit from Renoise (or a controller), here’s the midi documentation:https://d19ulaff0trnck.cloudfront.net/sites/default/files/novation/downloads/12118/circuit-midi-parameters-v4.pdf
And here some articles about circuit:http://cdm.link/?s=novation+circuit
Edited: to add some additional info.