Although I don’t generally like compressors as an effect, I like it sound just like my original amp but with the natural compression you get when you crank your amp up a bit.
This is me...let the amp compress. If you take the attack off your guitar before it hits the amp, you miss out on the amp doing that job for you in a more toneful manner. Don't really understand compressors I guess.
Compression is a hard tool to get your head around- there are a few really common issues/misconceptions that crop up
1) Compression doesn't have to affect the transient attack. This is where a lot of pedal compressors fall down. Having control over the attack or mix of a compressor is essential for getting the most out of them. In my experience, the Boss CS3, and Ross types typically suffer from this lack of flexibility. It's great if you're going for less attack/more sustain or body but misses out on the tone sweetening possibilities that a good compressor can add. Boss CS3 has an attack control, but still seems to hammer the transient pretty hard even with this wound right up (attack controls the speed that the compressor clamps down on the signal). The most classic compressed guitar tone would be for chicken-pickin' where you want a nice spiky transient, but also a bit of note bloom behind it so that you get a bit of thickness/richess to the tone and therefore hear the note clearly over the attack. Without compressor it can sound like clicks in a mix, rather than actually hearing the note clearly.
2) Heavy compression with a fast attack can make your guitar sound pretty lifeless- it tends to beef up the midrange and low end content which gives the impression of robbing your guitar of top end or definition. The opposite is true of a well set compressor- you can bring out the sparkle/presence of your clean tone in a really sweet way. A blend knob is a great alternative or addition to an attack control - you can blend in the unaffected signal to regain transient response. It can be fun to set the comp to SMASH and then blend it almost all the way back to dry so that the compressed signal is just lifting the tail/sustain.
3) Tube amps naturally compress so why would you need one? This is true-ish. The caveats are that every tube amp circuit will compress a little differently and typically only at higher volumes. For example, Marshalls tend to be far more linear in clean tone than Fenders, but far more compressed as they saturate, 6V6 amps have a similar response to a comp with a faster release so you get a kind of spike then bloom, and 6L6 amps tend to sound punchier. This is really apparent when you record a fender amp - the waveform tends to be really spiky. Finding a compressor that you like can really help get a range of tones out of your amp that tend to help it straddle those differences, but also get you a cranked clean tone at lower volumes.
4) They can be used to iron out the dynamics in your playing- this is true, but not necessarily in the way that you'd immediately think. Think of it as ironing out or shaping the dynamics of a note or phrase, as opposed to a whole song. Unless you set it to SMASH, you'll still be able to dig in to get a bit more grunt etc. It can be great for reducing pesky lumps in your playing though.
5) All of that being said, they can be really hard to operate because the aim is subtlety and there are a lot of variables.
Finally- almost every recording of a guitar that you've ever heard has probably got a compressor on it, albeit post amp. Optical compressors are great as they are naturally a bit slower and smoother, as well as more natural/transparent sounding- that's what I tend to favour for guitar or bass when mixing.
As a rule of thumb- I think they're more effective on single-coil guitars into a clean to slightly breaking up amp. I don't currently own one, but I've had a few with very mixed results. I'm keen to get a nice optical one again at some point, but it's relatively low on my GAS radar at the moment.
My picks of the ones I've owned are:
Durham Sex Drive - my favourite of the ones I've had- it's very subtle and when I had it, it was always on.
Visual Sound Route 66 compressor - I thought this was a really nice sounding dynacomp/Ross style one that could do subtle/sweetning without killing the transient.