Well like anything in life, you get what you pay for.
You can spend your life drinking Boone's Farm wine and Coor's Light beer because you are too cheap to pay for the good stuff. But if you decide to do so, you might save a bundle of money, but you will lose out on the pleasures of discovering and enjoying high quality wines and microbrews.
Well the same goes with radio. Yes, you can listen to terrestrial radio for free in your car but what you will get is close to 35 minutes an hour of commercials and radio DJ pitter-patter in the "morning zoo" vein, with their lame banter, pranks and jokes. Then you have the totally useless "traffic" and "weather" updates. In between, you might get 20 minutes of music which are the same songs you've been hearing for nearly two decades now.
If you listen to any kind of "rock" station, you are sure to hear "Wonderwall" by Oasis, "100 Years" by Five for Fighting," or "Don't Stop Believing" by Journey. If you listen to pop, you are going to get the Beyonce/Rhiannon/Justin Beiber/Jay-Z crap over and over again.
So yes, you can listen to radio for free if you want to.
With Sirius radio however, you have over 100 stations of crystal-clear commercial-free music in virtually any musical genre you can think of. If you are in the mood for old-time country music, there's a station (or two) for that. If you want to hear heavy metal from the 1980s, there's a station that plays nothing but that (Hair Nation). There's a station with nothing but Opera music. Or Jazz. Or Blues. And about a 30 different rock stations from classic rock to modern alternative.
What I like to do is capture songs I like on my SoundHound app so I can download the MP3s when I get home. They say no good music is being made anymore but they are wrong. There is tons of great music out there to be discovered. You just aren't going to discover it listening to "Wally in the Morning" on your FM dial.
I had a free year of satellite radio. It sounded overly compressed with strong digital artifacts.
I enjoyed the variety but the quality was lacking.