Yes it does eliminate noise when not playing but
will it get in the way of sustain?

Noise gates are stupid devices, so you have to tell them to do the things that you believe will not interfere with your playing.  For that, you have to know your playing well, so you can tell it what to do precisely enough.

They also behave best when they are almost unnecessary. If you have bad hum levels, the threshold for gating on will have to be set so high that you'll frequently lose the beginnings and endings of notes.  If all you have to contend with is fairly low hiss levels, where the difference between a played or decaying note and background noise is clearcut, they work great.

If you want to use a noise gate with pleasing results, the first step is to attend to all possible causes of noise first (e.g., shielding stuff that needs to be shielded, ground loops, keeping inputs and outputs away from each other in high-gain devices, shielding the guitar itself, use of HB pickups, etc.).  Once everything is confined to small levels of residual hiss, the noise gate can just do the aural tidying up, and you can set the gating threshold low enough that you'll never miss attack or note fade-outs.

There is one trick to improve working of a noise gate :)
Side chain part should be fed from input clean signal, free of hum and noise boosted by overdrive effects. And the muting part should be placed near the end of signal chain, somewhere before delays, reverbs.
Works great, I did it as one of upgrades in my H&K Cream Machine.
Regards
Peter

.... removing noise gate from my "to build" list

Thanks guys

Crawler:
Got to the party late, sorry.  In addition to all of the above, they mess with your tone--some more and some less.  I have used three: a Boss NF2, a Guyatone and an Alesis Microgate.  The Alesis actually didn't screw around with the tone too much, but I couldn't get it to gate effectively, either.  Maybe I didn't set the sidechain right.  It also added its own, subtle noise/distortion.  The Guyatone sucked tone big time--not worth the benefit at all, IMHO.  It is small, though  :? The Boss ain't bad, but it still affects tone, attack and sustain, and I decided to do without.  Best solution: kill noise at the source by shielding your guitar well, adding humbuckers or noiseless pups as needed, and cleaning up your pedalboard power supply.
Ben

I don't want to diss noise gates too much.  Rather, I think sometimes we have too high an expectation about what they'll do for us, and because of that they end up letting us down.  When you use them properly, adjust them properly, and set your expectations right, they do a wonderful job.  They are not miracle workers, though.

The downward expander on the SSM2166 chip, found in the Q&D compressor at Jack Orman's site (and occasionally available at Smallbear) can also do a very nice job.  Downward expanders accentuate dynamic differences below a given threshold.  Normally, gates work by establishing an output = 0 if the input is less than some threshold.  Downward expanders work by establishing output = input/<constant> below a threshold, so you still hear the signal as it trails off, but the last bit of decay will seem a bit faster since the drop in volume is exaggerated.

Noise filters, like the Rocktron Hush system, can also be effective.  These take advantage of the masking principle, in that hiss goes unnoticed when there is sufficient musical signal to distract from it.  As the audio signal decreases, however, the hiss becomes more noticeable.  A noise filter works basically like an autowah set for a very high rolloff range.  As the input signal declines, the filter rolls off at a lower frequency, eliminating the hiss.  This is often married with a downward expander to provide a one-two punch with respect to noise reduction.  Again, the advantage of a noise filter is that it does not chop the beginning or end of anything.  It just cops the high end of quiet stuff.  The caveat is that 60-cycle hum is down where noise filters do not go, so you need to use other tools for that.  The National LM1894 is a modestly priced dedicated noise filter chip.

Actually, though most folks here who dabble in designing things have focussed on noise-MAKING effects, maybe we should collectively turn our attention to noise-reduction once in a while.  A noise-reducing device that was optimized for guitar, in terms of both what it does and how you can control it, would be a nice thing.  Most stompbox noise gates and such are aimed for an I-don't-know-what-the-hell-you-plan-on-feeding-me input.  So, for instance, the attack/decay rates may be off somewhat, the threshold may be off, etc.  Something simple that provided 60hz notching,   tunable steep-filter lowpass for fixed hiss reduction, gentle volume attenuation (rather than hard gating), and an envelope-controlled hiss filter with time constants and range suitable for keeping audio grit out, might provide a nice one-stop-shop noise solution for many people.

Hi Ben.  How's married life treating you?  Drop me a line sometime.

Just finished the MXR noisegate from tonepad http://www.tonepad.com/getFile.asp?id=77 and it work good enough, I think. There have been mods to it since the first one. It now includes control for attack, release and attenuation which I really have no idea how good or will it really work. I just hooked it up as jumpers but I will try putting pots later on.

Have you tried this one?