Very good. Has multiple uses

It works quite well in the recording input for a CD recorder, to keep unexpected overloads out of the recorder. I also discovered it is very well suited for Amateur Radio receivers -- keeps the QRM and other static crashes out of the audio. Because most ham radio receivers don't have headphone safety limiters, the 166XS fills that need, too.

dbx 166XS: Great for streaming audio

I am using this unit to normalize the levels of a live Praise and Worship band with single people speaking on a microphone at church. I have the unit set for a medium attack, long release, and 4:1 compression and it works great. No additional noise or distortion. Now the band doesn't overdrives the recording equipment and the Pastor's sermon is boosted out of the background noise. I am using this unit to send compressed audio to the nursery, streamed over the internet, recorded to DVD, and transmitted over an assisted hearing system. What an incredible improvement to sending uncompressed audio!

Punchy

I'm an experimental artist that you might call "industrial". Using the gate on distorted bass drum combined with EQ and compression is amazing.
Now...all I need is to sample all my bass drum and snare hits and put them on a sampler...
It doesn't get a full 5 star rating from me for this reason: running a source in a serial fashion, with the gate turned up to maximum, I can still hear some audio. Not a LOT, but some leakage.

dbx 166xs: Home sudio workhorse

I am using it in a pair of channel inserts for my microphones in a Mackie SR 24-4. The -10 dbv/+4 dbu switch comes in really handy here, and unlike some other devices, it actually affects the level of internal processing, as opposed to just the meters. The controls are sensitive, and I can really hear the compressor attack and release time controls actually affecting the envelopes of the sound. The semi-detents on the knobs are a nice touch for repeatability. The auto setting for compressor attack/release also seems to work very transparently. The downward-expander type gate works great once dialed in. For me, it does not chatter like a switch gate. The LED metering for compression is odd and takes some getting used to, as the meters reflect the aggregate function of the compressor and the gate. But the logic is growing on me, and it allows you to see how the gate closing process proceeds as you dial in longer gate release times. It is a bit of a gaudy light show, however.

Best of all, it is absolutely quiet in my studio setup. Another big plus for me is that there is no audible distortion (I hate distortion in studio gear).

All in all I am very pleased with it, and, as expected, the silver faceplate is easy to read, and looks classy too!

compression

Im still learning the art of compression. This two channel compressor has everything I need to compress the signal going into my recording interface