Samson C-Com Opti: This is a one-channel (mono) half-rack-width optical comp, meant for use with a line-level balanced input. It's cheap, and it has 1/4" in/out jacks, so some people will try to use it like a pedal--but this is not a good idea. The input impedance is very low, so plugging a passive instrument straight into it will result in a severe loss of high frequencies and tone. Even an active instrument may not work well, since really this unit needs to be fed a higher-level signal.
Given the right input, it does not lose lows or highs. When the signal gets compressed heavily, the highs can get dulled/darkened, so they have included a button labeled "enhance" for bringing back some of the uncompressed high end. It is a bit subtle, but it does work. The noise level is quite low at most settings, though just as with any compressor, the more you squeeze the signal, the more any existing line noise will be amplified. The tone is mostly transparent: a little bit colored by the effect, but not so much that most people would actually notice.
The main thing you will hear is the action. It has an exaggerated squashy dip and swell, at anything more than the lightest settings. It has a very hard "knee", meaning the effect hits strongly the very instant your signal crosses the threshold. So the threshold knob will seem to be "all or nothing", where one small turn results in no compression or a lot of compression. You will need to experiment to find the right balance for your individual signal. The compression can get quite heavy, but it does not work well as a clean peak limiter for slapping. It has attack and release knobs, and they do exactly what they should, but they do not change the all-or-nothing action. The ratio knob is what you will use to dial the effect from the extreme squash down to a more natural, even, smoothing effect. So while I have described this unit as exaggeratedly squashy, really you can get much more usable, less obvious results from it; you just have to spend some extra time tweaking the threshold and ratio knobs to get there.
The construction quality is decent: not fantastic, but not particularly fragile either. It has a needle-style VU meter which can be set to show either the output level or the amount of gain reduction, and it is actually reasonably accurate and useful. The bypass switch is not "true bypass", but it is clean and effective. The unit is powered by a "wall wart" supply that puts out 18 V AC at 1000 mA, so it cannot be powered by any daisy chain supply, and there are very few other supplies that could do the job--so you are pretty much stuck with using the included wall wart.
The bottom line is that if you have a home recording studio, and just want to add to your selection of compression flavors, this is a tremendous value considering you can pick one up for $30-$40. I wouldn't recommend it as your only compressor, but if you're on a tight budget it would be hard to find anything better for so little money.