JLM Audio Equipment MAC Stereo Opto Compressor Rack by frans
I DIY'ed my unit as a mono 19" box. So what does it do? Fixed attack and release, with the usual slope of opto elements on the tails of compression. Gentle, goodnatured and easy to use. Which makes it good for tracking while you got to watch ten other things at the same time. You can cut some lowend from the sidechain (@100 or 200 Hz) so the signal doesn't get pummeled if you do drums or bass, for example. If you don't slam it too much on the wrong sourcematerial it helps the tracks to be mix-ready. What doesn't it do? Anything you'd use to completely warp the envelope of the dynamics. Also not a thing that fits 100 different tasks for compression, just doing one thing oh so right.
Sound quality? First class. I have seen the inside of a few other opto comps that use about the same topology and at least some of the same parts and they cost more, so don't be fooled by the low price. By the way I nicked the photo off the JLM site.
As a DIY build it's not too complicated, but that depends on how many things your soldering iron already did. The build thread is very clear, step-by-step and Joe is a saint concerning help with his builds. Also the community on the JLM forum is very helpful. Bang for buck is 10 out of 5.