User Interface:
Well, compared to other plugins it does look a little soft and washy, but it doesn't hurt the eyes like oh-so-many others.
The controls are laid out logically, from input gain on the left over FX controls, the visualization and post-FX enhancements to the output volume and a dB meter on the right. The way it should be.
There are lots of readouts, all controls (apart from visualization, zoom and meters) are explicitly labelled with intuitive names.

Sound:
One word: transparency.
I never knew what that meant when I read it. Since I have this plugin, I know. This plugin does affect the sound, as it will tame the dynamics and peaks/transients, but it doesn't add any "color" or "warmth" and doesn't try to add any harmonics, artifacts or other "analog sound". At least none that I could hear.
This is a compressor and a limiter, you use it to really squash signals with it. So if pushed to its extremes, it SHOULD clip and distort the sound something rotten, as distortion is basically nothing else than a dynamic range that's too limited to reproduce a certain level and therefore "cuts the tops off" and makes sound waves increasingly rectangular. But you can mangle signals through this plugin as hard as you wish, and it will still sound good, introduces no to hardly any audible distortion.

Features:
It is actually kind of a one-trick-pony, but I value this as being positive.
There's no need for additional or extended functionality, it does what it says it does - and it does it really well. That's good enough for me.

Documentation:
Let's call it "sufficient". It comes with a 4-page PDF, two pages are taken up by features and the version history. The remaining two pages concentrate on the controls' functions and how to use the plugin. Pretty insightful for anyone with a limited/basic understanding of dynamics.

Presets:
It comes with the usual "Init" reset-preset, with 4 Mastering-presets and an additional that's called "Violator". :)
I'm giving 9 points here, as it does have little presets but in my eyes there's no use for presets at all.
This is a dynamics plugin, it reacts to a track's volume and changes in volume, so it's hard to make useful, universal presets for it, as all tracks have pretty varying dynamics. I'm not much of a preset guy anyway, so this is actually pretty irrelevant to me. 5 presets or 50 ... doesn't make a compressor good or bad.

Customer Support:
I have had eMail contact with Stefan, the developer, in the past, he seems to be a friendly guy who knows his stuff. He answered my questions fast and extensively, that's more than most of the "big" companies have to offer.

Value For Money:
Unbelievable. I've been using this for about a year now, and PeakCompressor is one of the two or three plugins I actually don't regret paying for. It's just SO handy.
Even though it seems it was intended for Mastering purposes, this is so CPU efficient that it can easily go on every track without generating a significant load.
And that's actually what I do with it, no matter if it's a vocal track or a bass/guitar DI track or whatever... if there are any occasional peaks in my recorded material - I use PC to squash 'em and raise the overall volume!

Stability:
Never crashed once in the year that I've been using it. Works well in whatever environment I used it, be it REAPER or Wavosaur.

Conclusion:
Could not live without it! If I had to, I'd pay for it again.

I have been a fan of this compressor for many years. Sinus has been great about feature requests that I have sent him. It's a shareware item, so you could use it without paying for it, but why not just buy it? It's very inexpensive and the only way we can keep good developers producing great products is to encourage them with our money. That's what I have done. So, what's it sound like? It really sounds like nothing and takes almost no CPU. It is a very transparent compressor that reacts to peaks using a "look-ahead" process. It's not a tone shaping compressor at all. I select PC when I love the sound of a track but need it to be a bit louder and tame the peaks. It does that flawlessly. Because of the attack speed, PC can be used to smooth out sounds that contain too much attack and not enough body. It's really quite nice on acoustic guitar and voice. It's not my first choice if I am trying to color a track, but to simply control peaks and get more body from a sound, it can't be beat. It also has a brick-wall limiter and dither integrated into it. I believe that Sinus intended for this to be a full mix finalizer plug, but I have used it more on individual tracks. By taking the release out of auto mode, you can get some pretty huge sounds from percussion sources. Another trick is putting PC after another more colorful comp. The first comp shapes the sound and PC evens out peaks and brings up the body. Follow that up with a good EQ, and you have a seriously powerful setup.

The UI is great. You can easily see exactly where your compression and limiting start. His wave display is wonderful and unlike so many others, it doesn't cause a huge CPU hit.

I gave presets only a 9 because I honestly never even used them. PC is intuitive and takes no time to master. It doesn't do everything (nothing does) but it does what it's supposed to do very, very well.