I haven't, so take the following with a large pinch of salt.
Systema to me strikes mje as extremely dangerous to train in if you've never done any form of self defence training before. The reason for this is that there are a lot of very dodgy looking principles backing this art (watch some of their knife defence videos on youtube and you'll see numerous demonstrations where the weapon arm isn't engaged at all or where the knife is deflected by simply wrigling the body or arm in its general direction), principles which could easily get you killed if you buy into them and only ever train in a compliant manner.
The fact that so many of their demonstrations appear to be nothing but gimmicks really doesn't inspire me to try this martial art out.
That said, I'm sure there are instructors out there who take it seriously. I just don't like what I've seen of it to date.