Hi Andrew, I recently purchased a Scarlett 6i6 and have only begun using it. Here is what I've found so far: I like the preamps - I find them clean, quiet and not harsh sounding. Buffer Size: 1 msec, 45 samples; Asio reported round trip latency is 7.8 msec, 343 samples. I'm still testing the system and so far I've only recorded 7 audio tracks with plugins (vocals, guitars) and two midi instrument tracks, so, very early days. I've no problems with latency so far (see also computer spec).
I very much liked the bundled software, especially the Red 2 EQ - very clean without colouration. Unfortunately, it loses it's settings when you exit Sonar and boot up again. Focusrite have still to come back with a solution but in the meantime, a workaround is save settings within Red 2 to a Bank file and then open the file when you next use Sonar / Red 2 (not very handy but better than nothing?). Also, acoustic guitar sounds good with the Inst setting engaged but there is a high pitched interference in the background when using electric guitars (Les Paul) or electric bass (p-bass). I've tried the electric guitars through another preamp and there is no problem with the guitars. It doesn't occur with the computer switched off. Mic's work with no problems and the Line inputs are fine also. Focusrite have still to respond to my query regards this anomaly.
There is an excellent video by Mark Warren on YouTube for Mixcontrol here:
https://youtu.be/VDo7-fzzKtYHope this is helpful.
John.