T.C. Electronic Sustain + Parametric EQ: This review is of the original (discontinued) version, not the reissue. This pedal doesn't do everything you might want from a compressor, but what it does do it does brilliantly.
There is no loss of highs or lows, although the high end does sound kind of dark and sometimes a bit muffled. The low end is big and full. The tone overall is warm yet clean and not very colored; it has a "soft" or "smooth" quality, as opposed to crisp. There is almost no noise. If you crank up the sustain knob then of course the background noise of your signal path will be amplified a lot, just as with any other sustainer, but even then it really doesn't sound as bad with this pedal as the noise from most others. The ratio is fixed, and is very high--I'm guessing around 20:1. That makes it great as a sustainer and a peak limiter, but it also means this pedal won't do subtle, invisible, or gentle compression. Part of that is because the threshold is very low. The sustain knob increases the gain of your signal past the threshold, and I found the only not-totally-squashed setting was with the sustain knob turned most the way to minimum. The good news is that if you like Tony-Levin style heavy compression, this one does that job with cleaner tone, fuller low end, and lower noise than the other squashers typically used (including the Analogman that Tony uses). Also if you just want very consistent levels without too much alteration of your tone, this is a good tool for that job, when the sustain is set to the very minimum. You can effectively raise the threshold (getting less squashed) by using a lower-output instrument.
The distortion switch is very different from what I expected. It's not a buzzy or gritty distortion at all, and in fact at first I thought it wasn't working. What it does is adds a very subtle "fur" so that your tone sounds like it's wearing a fuzzy warm scarf. It has a soft "vintage" quality, and it's a neat effect that you won't get from most other overdrives. The EQ is excellent. It's only one band of EQ, but if you just need a little corrective bump or notch to perfect your tone, this one does that job well and cleanly. I got equally good results bumping the low mids for a fat reggae tone, bumping the high mids for a more lively and cutting tone, or notching around 1K for a bit of the SVT scoop.
The noise gate function on this pedal seems to be broken--the knob doesn't do anything. So I have no idea how well it would work otherwise.
For power it takes a male 1/8" DC plug, not the usual Boss barrel type. The footswitch is not "true bypass", but the bypass is quite clean. The construction quality is a mixed story. When these classic older TC pedals are working, they are some of the best-sounding pedals you can find; but when they break, they are nearly impossible to fix, and TC charges a minimum of $100 USD (including postage) for repairs. I have now tried to make simple repairs on the Sustain+Para and the Chorus, and both times I had to give up in frustration. I sent the Chorus in to be repaired by TC, and they replaced the whole circuitboard; but then I mailed the pedal to a buyer in China, and it arrived non-functional. As much as I love the sound of these pedals, I don't trust their reliability. If you're a touring pro, bring backups. I just bought a second one for a backup for myself, and it arrived not working.