I had a Gran Prix I bought in 1973. For me, it was a great bike. I rode all over the place on that - back and forth to college, on tours, etc. However, in hindsight it had serious flaws. The wheels were crappy, it only had 10 speeds, it was heavy, it had cottered cranks, and a big bolt sticking up on the top of the stem. The Simplex derailleurs worked fine, but they sure weren't index-shift.
I'm so glad I finally graduated from college, got a job, and could afford a better bike. I loved that old bike, but I wouldn't want to go back to it.
However, for $40, if you get a fraction of the enjoyment out of it that I got out of mine, it's a great deal. If you're a starving college student, it might be just the bike for you. If you have a couple hundred dollars you can do MUCH better.