I have no idea the kind of voodoo magic this plug-in has in its coding, but it just sounds incredible. For such a simple compressor it is actually incredibly flexible. It can be used on just about any thing and sound great but the key is to know how hard to drive it. It is this point that really sets it apart from a lot of compressor plug-ins, but to really make it work requires using the Hold and Release knobs to get the release character of it to work.
For example just light touching it on a vocal will add a nice level of character to it. Typically pushing a vocal really hard into it doesn't sound all that great, but using the "smoother" option can help if you require a bit more compression. Another personal favorite position for it of mine is on toms. Just ratchet up the compression level and listen to those toms poke out in the mix. Where it really excels at though is on bass and buss duties. It can make a bass so solid in a mix that you'd swear that is how the bass was always supposed to be. If the bass needs some more harmonics just drive the compression level up, and I do mean up. Don't be afraid to get 15 - 20dB of Gain Reduction going on it.
Now, the buss level is where I find this thing really works magic. Drum busses can typically do with a bit of "glue" so to speak. I typically like bringing GTS-39 right up to the point where the drums start to break up a little bit. Kind of at the point where GTS-39 starts to give out a little bit. Back it up a little bit and set the output level appropriately for the mix. The drums now have a wonderful character that just works.
The final place worthy of a great mention is on the Master buss. Typically just lightly touching the compressor and having it compress in time with the track will simply work wonders. I'm not talking about a lot of compression either, 2 - 5dB at most. It "warms" (I dislike that descriptor myself) the track right up. Everything just gels together in such a way that it can only be magic.
Of course there are a couple of other nifty little features. The Side-EQ can be wonderful if you're having issues with the low end causing too much compression and the ability to set it to a Dual Mono unit. Very handy stuff there.
The only real two complaints I have about it are: 1 - The release knob needs more flexibility. Normally, I'm working in the very bottom percentages of it. 2 - Control over the attack. Even if it were to only increase it by a few milliseconds it would be awesome!
On the whole though, this is kind of like sprinkling magic fairy dust on a mix.