Summary: This is not a game. This is a simulator. And it server perfectly well as a simulator. Mistakes are costly in racing and iRacing makes sure you pay for them. Even though the penalty system is penalising both drivers, it teaches you to look in the mirrors and actively try to avoid danger.
Summary: the most serious recreation of motorsport you'll find. the sim boasts the most accurate circuits we've ever seen on PC - the proprietary laser scanning system does an incredible job, and you'll discover bumps and undulations that simply aren't there in sims using handcrafted tracks.
Summary: Simply no comparison, by far the closest to racing that you can get, only limitations will be yourself and your equipment. Remember this isn't your console controller based racing sim. Be warned, this is a racing environment that is taken very seriously.
Summary: Excellent sim -- simply the best out there. The amount of work that has gone into making iRacing a realistic experience is truly astounding. I've been a racing sim devotee for 20 years and iRacing is top of the pile.
Summary: Absolutely, 100%, hands-down the single best simulation racing title that has ever existed. It is extremely challenging to adapt to, especially if you are new to sim racing. But the robust online community allows adapting to the game seamless.
Summary: This sim is quite expensive, so you should focus on one class rather than buying all cars - well if you have the money why not, sometimes you really have to try everything ! And this sim is so close to perfection, you really want to try each car and track.
Summary: iRacing claims to be the best racing sim out there. It surely has plenty of tracks and cars. However, there are many things about iRacing that are not so great. The game is setup with a subscription service. Already it's annoying because it does not fit the subscription based game format.
Summary: By far, the best racing simulator I've ever "played". Not only the physics, car response and tracks are brilliantly simulated, but everything that involves competition (points system, race splitting, safety rating...) really works, and make you feel close to real live racing.