Supremely addictive.
I've been racing sims since the mid-eighties, when you were lucky to get a few jagged lines on the screen to denote the track. I have pretty much every PC racing game out there, and most sit unused. Racing against AI is dull, driving hotlaps is dull, and most online races are crashfests or completely devoid of competitors.
I'm a recent iRacing convert, and the ability to jump from one race to another, against decent sized fields, even in the Australian timezone, is a winner for me.