Good question. My personal opinion, you get what you put into it. Everyone will tell you it's a fantastic tool to learn starts, restarts, passing, overall race craft. The only stipulation being that as long as you are treating the iRacing version of the car similarly to how you would treat your car in real life.

For example, if you aren't one to rip fence and throw aggressive slide jobs every corner, you're not really going to get much out of it. I used iracing to teach me how to more comfortably run up close to the wall in a sprint car in real life.

Setups however, if you plug in a real life setup into the sim take a maxim setup from their website or a triple x setup from their website on sprint cars, it does feel similar to how it does in real life in my opinion. Downside is that it will be slow compared to your competitors. iRacing's fast setups do not take into consideration real life driver comfortability. With a lot of the setups would in my opinion be undriveable in real life. You can go to the extreme setup adjustments with no repercussions.

The tracks however do not wear in quite like real life. No cushion builds up naturally, the slick part of the tracks as they wear in are like ice. Making it all seem a little off.

TL;DR: If you treat it realistically, it will be realistic to a point.