Well, I used it for a class, and it was fine for class purposes.

I did notice significant differences in modeled circuit performance between versions of Qucs Studio, for even very simple models. Instead of trying to get to the bottom of that problem, I just used the same version as the instructor and called it "good enough."

But "good enough" for a class is definitely not "good enough" for the workplace. Either I was doing something stupid (quite likely) or you should stick with ADS.

I haven't tried QucsStudio but Qucs has worked when I tried it. It's too bad development on Qucs seems to have stalled.

My favorite fast, simple, and free linear simulator is still Puff: https://www.pa3fwm.nl/software/puff/

It works quite well for linear network simulations (eg: plugging together black box S-parameters from manufacturer data or extracted from a VNA) and it works reasonably well for filter synthesis. I've never used the harmonic balance simulator or the new planar EM sim though.

not tried myself but here is some documentation: http://www.gunthard-kraus.de/qucsstudio/index_english_qucsst...