I read things completely wrong in these opinions.
A compressor is indeed difficult to understand! That is the role of the buttons: <ul>
level is the level of output after the effect (the master of sorts)
tone is a course correction bass / treble (like radio)
attack is the attack time. The longer this is maintained over the initial attack before compresses
Sustain is the gain before the compression.Plus we put, the more you will have the sensation of "pumping" sound.

UTILIZATION

There are many different uses. <ul>
synth: 0 attack, lots of sustain, chorus behind: the sound will be compressed when the attack
may be required to submit with the Treble tone. The breath is inevitably raised.
clean / half blues attack, sustain the same, then there are the notes ataque while controlling aggressive peaks. Low drool least, the highs are more mellow.
OverDrive one before it can be used to boost the gain and sustain to have "infinite"
compreseur goes before the amp in 99.99% of cases if it is useless
before a very large saturation it will not help the already saturated pad!
When the amp is very loud, the compressor may become useless.

SOUND QUALITY

We must spend time to find the setting.
On stage settings will certainly be a little different.
The sound is OK, without charm but does not alter too much the guitar either.

OVERALL OPINION

I have the BOSS and Dynacomp. On the latter lacks the setting time of attack.
The BOSS allows me to return home without playing a punchy sound too hard. My amp on stage compress itself not bad, I see less utility.
The value for money is good (bought used)