It's excellent. I use it in almost every project. I've used it on a wide variety of sources, from drums to vocals. Many overlook its extensive modulation capabilities, which let you do interesting things such as making a fingered bass sound like it was played with a plectrum.
However, Saturn is not the only harmonic distortion plugin that I use. Different tools for different scenarios. Saturn does really well with
subtle distortion (great for brightening acoustic guitars, for example), but its more extreme algorithms are beyond anything I'd ever use. So I only use the tube and tape algorithms, and at gentle settings.
My most-used distortion plugin is the standalone Exciter module from Ozone Advanced. Unfortunately, you can't buy it separately so unless you're ready to invest some $$ in Ozone, it's not an option. But it is the most versatile harmonic exciter I've ever used. Doesn't have Saturn's modulation toys, but it has a wider range (for me) of usable distortion models.
Another favorite, although limited in applications, is Redopter from D16 Group. I use that one in parallel, most often on a sub-bus that I mostly send drums to. Especially good on kick and snare.
For more extreme distortion, especially on bass, I often turn to Voxengo Boogex. It can give you anything from barely-detectable to full-on dirt. And it's free.
Don't forget amp sims and stompbox emulations. They're not just for guitars! Run them in parallel on a rock vocal, for example, to add grit.
Then there are the hybrids that happen to produce distortion along with their primary effect. Cosmos, Bus Driver and RBass come to mind. Sometimes, they deliver all the distortion I need even if that's not their main purpose.
One distortion plugin I
never use is iZotope's Trash. They were clearly aiming for an entirely different market segment with that one. But it remains in my inventory, you know, just in case.