I started with the original Assetto Corsa doing online lobbies in the mx-5 at Silverstone, great fun for less than a fiver. Can't comment on ACC. Difficult to guage if I was improving though to be honest.
It's possible to pick up 12 months iracing plus the FIA F4 car for circa £25, so thought I'd give it a shot.
It's basically impossible to go to any other racing 'game' now for me. I just used the free content initially, fell in love with the MX-5 and all the rookie tracks. Started purchasing some of the paid tracks for use with the F4, and now there are several cars and tracks I love driving.
The ranked races, the safety rating system and the ability to protest other peoples carelessness is all worth the money in my opinion. It creates the illusion of a real race with jeapordy for your actions.
It's the closest thing you can get to the thrill of real motorsport competetiveness in a video game if you ask me.
I'm hooked and if I ever let my subscription lapse, I know I'll be an iRacing customer again in the future. Being able to race at any hour of the day in such a polished enviroment is mind blowing to me.
Of course, there are negatives too, the cost being prohibitive if you want to venture outside of rookie content.
For example, I've just made the decision to stay using my T300 wheel for another year, and spend some of the money I was thinking of putting towards a Direct Drive wheel on Cars/Tracks instead.