I've owned or still own several 500 series comps. I've used the Buzz Essence, the Purple Action, the Alta moda AM-10, The Pico, and the Pete's Place
BAC 500.
BY FAR my favorite of all of them is the Pete's Place BAC. It's extremely versatile. It's fantastic on bass, vocals, and really excels on acoustic guitars. I've also used it on electric guitars. There's nothing I've tried it on that didn't sound great.
The Buzz Essence is a fantastic comp. It's pretty clean, but does add a little bit of sweet honey sheen to the tracks. Sounds great and it has high and low filters on it. Definitely my second favorite. Really good on vocals and bass.
The Purple Action was by far my LEAST FAVORITE. Never could get it to sound really good on anything. The knobs are small and difficult to turn. The meters are next to useless. I sold it pretty quickly.
The Pico is by far the cleanest of the bunch. What goes in, comes out sounding the same, just smoother. It has Peak and RMS modes too.
The AM-10 is very tweak intensive. Lots of settings with limiter and comp. It is not a set it and forget it kind of comp. The limiter function is pretty useful. I sometimes use it in front of the BAC. It also has a wet/dry mix option which is also useful.
Do yourself a favor though and try the Pete's Place BAC. It's a fabulous piece of equipment. A couple of those, and really, you wouldn't need much else.