Always on.  That's how I tend to think of a great compressor.  Personally I won't use a compressor that doesn't allow me to leave it on all the time.  That doesn't mean I've got the thing fully cranked.  I'm looking for some gentle compression, some additional sustain without boost, and no tone colouring.Empress Effects is a Canadian company, founded by a graduate in Electrical Engineering from Queen's University, named Steve Bragg.  Since founding, Empress has grown but is still small company staffed by people who live and breathe great effects pedals.For years I have recommended the Diamond Compressor (and still do!) but I wanted something more like a studio compressor that I used in another life.  I wanted control of compression level, attack, release, plus the ability to control the mix of the compressed and uncompressed signal.  Different guitars are different.  Pickups respond differently and we know that different pots have impact on the sound.  Then we plug into different amps, be they valve or solid state and they all have their own sounds.  Push some valve heads really hard and they naturally compress.  Think of a Marshall 100w head when cranked, but do it quickly before you go deaf.  Turn a really great solid state amp up and it just gets loader.  An example here would be the still brilliant Roland JC-120 Jazz Chorus which does a nice job of holding clean tone while it attains disturbing the peace levels of volume.  Combining different guitars with different amps and you have a lot of variables.  This is why most studio recordings use compressors all the time.The Empress Effects compressor is the finest stomp style compressor that I have ever encountered.  And because I am a geek, I have tried a lot of compressors.  The very famous Ross design compressors are popular with lots of people, but I don't like them.  I don't want a compressor that sounds like a limiter or a dynamic range squishy toy.The Empress fits my use cases.  I can select from 2:1, 4:1 and 10:1 compression ratios.  Some folks may prefer a constantly variable level, but practical experience tells me that I prefer consistency.  I tend to run 4:1 most of the time, but it will be dependent on the combination of the guitar and amp.  I also like the meter which basically can show you the input, the output or the combination.  It shows you what the compressor is doing in real time and is a really great way to see live if you are over squishing, which may sound ok in one place and sound like cat yack elsewhere.In addition to the controls for input and output, you get very effective controls to set the compression attack and compression release.  There is also the Mix control to allow you to determine the blend of dry and compressed signal.Set up is a breeze and you don't need an engineering degree to get going.  The only piece that may be a bit confusing is the Sidechain jack.  This is a TRS connection that allows you to bring in another device (it's a loop) such as a high pass filter that you might want to use if you are concerned about very low but high energy bass notes tripping the compressor.  With the growth of 5 string basses, this can be a very useful tool.Generally a compressor goes very early in your signal chain, and definitely in front of the amp.  In my case, it goes first after the wireless receiver, and then to overdrives etc before they go into the front of the amp.  However, it's your tone, try the compressor in different places until you find the sound that appeals to you most.Why I have chosen the Empress Compressor is because it NEVER colours my tone.  I can push it hard and I have to really do so before I get clicks or other compression artefacts.  It is so much a better experience than TC's respected HyperGravity which I returned because it clicked so much even before I got to the level of compression that I wanted.I have made note of settings for different guitars and different amps, because low output single coils into a sparkling clean amp will benefit from different settings than the PAF want to bes in my R9 into a Silver Jubilee.  What makes the Empress awesome is how versatile it is.  If I did star ratings, this would be a 5 out of 5.  If your dealer doesn't carry Empress, ask them why not.  Admittedly they are not the cheapest thing out there, in fact they may be seen has a premium brand.  Fair enough, premium sound for that price.