Setting aside all this OTM vs NTM stuff, I will say personally, no I don't feel iRacing is very realistic in it's current state. It's miles ahead of many other games (GT5 and Forza especially) but it's still not very realistic.
Granted, I have never driven any of these cars in a racing environment in real life. However I did grow up racing Formula Ford and Formula Continental. I can absolutely say that if I try to drive any of the cars in iRacing the same way I drove either of those cars, I would be even slower than I am now. iRacing is awesome IMO, but there are plenty of real current professional race car drivers who play it who are dog slow, and some who are fast as well.
People get too caught up with "well the car shouldn't do that!" but the fact is, in iRacing, in its current state, it DOES do that, so you must adapt to the iRacing style. I'm not a game programmer, so I can't say how difficult it is to program an accurate physics engine and tire model, but it's obviously pretty damn difficult because we've been working on it for 20+ years and still can't get it right (at least not at the consumer level).