I couldn't agree more with
@RasmusP, in fact I'm considering to become a premium member of RD myself. That said, let me talk a little bit about my single player experience with AC...
There are roughly two aspects about single player though, hot-lapping and racing against the AI. IMHO AC is perfect when it comes to the first one, you can go around the circuits trying different cars and driving techniques, improving your times and skills in the process. I have the Ultimate Edition, therefore a lot to choose from with all those DLCs. One day I'm driving cars like the BMW M235i Racing which are awesome to learn, the other I'm driving GT3s, GT4s or simply road cars like the Ferrari 458 Italia or the Mazda MX-5 ND and Cup edition. I never get bored nor disappointed when I use AC for practicing.
Now, talking about racing against the AI, with all honesty, although it has improved tremendously, it's still not as good as the AI in rFactor 2. If you really want to play against the AI in AC (or against humans for that matter) I suggest using the Helicorsa app, as sometimes it becomes difficult to avoid contact with them. They race you hard and it's amusing with the right settings, the only problem I see is a lack of spatial awareness in tight situations, although that depends a lot on the track you're racing. It's a bit hit and miss. rFactor 2 is a world apart when it comes to AI and spatial awareness, it's almost like racing against humans.
So in a nutshell, my single player is like this: AC for hot-lapping and rFactor 2 to race against the AI. Now, this could change once I find decent servers to race AC online and subscribing to RD could be the best way to achieve this.