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.