I give a 10/10 for mixdown purposes, and that's what I use it for.
The GUI is awesome and self explanatory. It gives you a very good view to what it does to the signal. But if you prefer to be not distracted by your eyes, just turn the graphic off.
Pro-L gives you an input gain, turn it up, lean back and watch the cool graphics and it will limit at the ceiling you set with the out gain in the right bottom corner. It's as easy as that and works almost all the time for single channel processing.
For further processing, e.g. for stereo signals, busses or even master, you can change the algorithm (called "Style"), lookahead, attack, release and how transients and release influences stereo signals (see section "Channel Linking").
Especially for mastering situations you can choose oversampling, dither, noise shaping and ISP (inter sample peaks [protection]). And if that's not enough just assign a MIDI controller to any of the parameters and tweak Pro-L for creative uses or simple automation draws.
I use Pro-L for channel processing only, when the signal has a wide dynamic range and needs more than compression to bring it up in the mix. It saved my life many times.
The first thing you notice is the amazing display. Looks like blood dripping over icebergs. The dripping red indicates the volume being squeezed off the peaks. You can even roughly "tune" the limiter by aiming to get the reduction (red) to neatly fit the peak (light blue). The attack and release correspond well to the display, but I couldn't notice any visual representation of the lookahead. I can hear changes as I move the dial but can't notice anything on the display. Guess I shouldn't ask for too much.
The operation can be divided into two stages. Firstly the transients are detected and squashed according to the lookahead and "style" values. Then the limiter applies "attack" and release to the result. Calling this first part of the envelope "attack" is unnecessarily confusing. It would probably be better to call it "hold" or "delay". Measuring the attack in seconds is also pretty confusing at first. Thankfully the display gives you a good idea of what is going on.
There's also the option of separating the channels (0-100%) for both stages of the limiter. I haven't explored this feature yet. It's making my head hurt just thinking about it.
The "style" variations do genuinely sound quite different, especially at high gain settings. With all the options it's hard to imagine not being able to get a good sound out of the Pro-L. You might find you can comfortably squash 10db off your tracks with this thing.
What else can I say?
It's got oversampling. It's got dithering and something called ISP which you probably don't need.