Aye to gkinsingapore... I find compressors a very useful tool when recording clean parts. Indespensible? Well, no. But extremely useful.
Its not about the technic all the time... at the end of the day, there's the short coming of some things. Like when you play clean, with .09 guage strings. You find your steel strings painfully tiny and wimpy sounding when compared to your wound strings. There, a compressor works WONDERS!
For distortion, if you put it BEFORE your dirt pedal or amp, you find you get tighter chug, and your chords retain more clarity, every note seems to stand out better. Put it AFTER your dirt pedals, or in the efx loop, its a different tone, a different function, esp since it evens out the vol very well.
Finally, whatever the comp you choose... note their pros and cons. I didn't take a DynaComp because I didn't like the why it made the tone all thick and creamy. It also squishes like no tomorrow... worked well to make my .09s sound better for jazz wannabe playing but not my type.
The ED-1 has been getting good reviews, and randolf has a mod for it... I can't rem what type of comp is it modelled after.
I had the Randofled CS-3 for a long time... Randolf does very good mods for it.
Finally settled on a local made Optical comp, called the Citrus.