Do I need a compressor?

Voxman

Very rarely use a compressor for the reasons already stated but it has its use for certain tones. 

Beexter

I couldn't get on with compressors until I tried one with a blend/ mix control. For me, that's crucial to get the benefits of a compressor without the loss of dynamics/ pick attack.
I've ended up with a Suhr Koji which I really like. 

heavyrocker

No, but you definitely, definitely need an R8. 

Rowby1

Beexter said:

I couldn't get on with compressors until I tried one with a blend/ mix control. For me, that's crucial to get the benefits of a compressor without the loss of dynamics/ pick attack.
I've ended up with a Suhr Koji which I really like. 

Same here, played for years without a compressor. Now got a Cali76cd on my board and I don’t know how I managed without it. Blend/mix is the thing that really makes any compressor much more useful.

Grunfeld

Blend does it for me too.  [Wampler Ego]

Depends on the guitar too:

I love playing a Jazzmaster live but sometimes I want it to sound more like humbuckers and a compressor does that.

pmg

I’ve had one on my board for a good 8 years or so (diamond comp).  I use it with a Ric for arpeggios and solos

gibsongretschfan

I use compression for funk rhythm, single note popping lines, most pop/pop rock tracks, U2 type nonsense and some fusion

I mostly turn it off when using large amounts of gain, are playing ballads where there is more dynamic space available to the instrument or I want a little more breath in the guitar sound

joetele

I've often considered getting one because I use so many pedals that sometimes get a bit wild at certain settings, but also to bring out the picking on my telecaster a bit and ensure everything is level. 

When I was a young fledgling guitarist, I had a Boss one which was useful for sustain, or tightening up the stupid nu metal chords I used to play. Thankfully I've moved on. 

Alnico

I use a compressor within the GT100 sometimes with the strat and the p90 PRS but not that often.
I don't think I've ever used it with humbuckers.
90% of the time for playing live I don't need it and I prefer the more open sound without it.

It really depends on what I'm playing.

ESBlonde

The style of music you play and your own playing technique all dictate to a degree how effective compression will be live.
If you have poor technique they make it sound worse.
It pays to understand what a compresser actually does and get a feel for threshold, knee, ratio, make up gain and why/how noise seems greater and treble seems slightly less in typical use. Full on studio compression is fully variable and can be used creativly whereas most 'guitar pedal' compressers are preset for one of two typical guitar style compression envelopes. There are a selection of online resources that explain the detailed working and setting of compressors
Mostly you will find it hard to hear a well set compresser being turned on, more likely you will notice when it's turned off after you've played through it for a short period.
If you get 'pumping' it's because one set of frequencies, the bass probably, are dominating the rest of the signal. Multiband compression as used in mastering and broadcast is used to overcome this usually undesirable trait. Jut to complete this 10p rush through compression you can also make a compresser squeeze the signal triggered by another signal. It's known as ducking in DJs where the voice compresses the recording while the DJ spouts over your favourite bit before returning it to full volume. It is also used so that the kick drum can duck the bass guitar to provide the percussion of the drum then the swell of the bass note in perfect time with one another.

Now do you need a compresser?

Musicman20

I've recently been trying out the Xotic Sp Compressor more, and it is addictive...I must admit I really like quite aggressive compression.

From another point of view, as a long time bassist I can also see the importance on bass!