I have a pair that I'm kicking the tires on right now. I've found them to be very versatile. They can be set to be smooth and transparent all the way to aggressive. The variable feedforward and feedback control is a handy feature. You can dial in a snappy and fast SSL type with the FF, or go more rounded and slower with the FB, or anywhere in between.
It took me a little while to get used the relationship between the ratio and threshold controls. Both affect each other unlike most compressors I've used. Dan Kennedy can explain how they interact better than I can. In my use, I've found that setting to what you expect to be a fixed ratio changes depending on where the threshold is set. It actually forces to you listen and make changes, which is what we all should be doing anyway.
You can dial in a very specific compression, or even peak limiting depending on where you set the controls.
So far, I've used them on vocals, bass guitar, acoustic guitar, and drums. They have excelled on all of them.
I have a Crane Song STC-8
in my arsenal. That beast is a PWM sidechain as well. The Crane Song usually sits on my mix bus as it's the most transparent compressor I've run across. The Great River PWM-501's seem to be more versatile with it's range of settings and sounds. I almost never use the STC-8
for tracking. I like to get a little color going on the way in. I've found the PWM-501's can do both well.
I will be buying this pair for sure.