I use this plugin to print subtle mojo to recorded tracks. My microphone selection is severely limited, so I find myself using an SM57/SM58 for most tasks even if they aren't necessarily the best mic for the job. I've found that if I throw an instance of Red EQ on, say, a vocal or acoustic guitar, and dip the mids by about 1db, and boost the highs by about 1db, and adjust the lows to taste based on the source material, I get a sense of brilliance and clarity that wasn't really there (or was just hiding). Boosting the low end with this really gives a sense of character that I haven't been able to get with an algorithmic EQ plugin, and while I'm sure I could get there eventually, I like that I can just throw this plugin on, slightly tweak one or two knobs, and get a nice analog character with very minimal effort. I like very specific tools with only a couple knobs, and this fits the bill.
I don't really use this tool to mix, because of the limitation of frequencies, and because it kind of bogs down my system if I have more than a couple instances running. So I just dial in a really subtle sound that I like, and print that to the track. In that sense, it's kind of like working with hardware, since I'm committing to the sound and moving on with the song. This plugin is more of a "mojo" box than a mixing tool, and while it can be pushed to extremes while still sounding great, that's not really how I choose to use it. That said, try cranking up that high end; it's really quite smooth.
I also make sure to delete the instance from my plugin chain when done, rather than just bypassing it, because each instance seems to add about 5 seconds to my project load time if I don't. Also, I've found that (in REAPER anyway) copying one instance to another track takes several seconds longer than just adding a new instance. Don't know if this is a REAPER thing or an Acqua thing, but the first time it happened, i got really nervous because my DAW became completel unresponsive for about 10 seconds.
8/10 because of latency, load time, and limited usefulness, but I love what this does to my vocals and acoustic guitars.