Trust me i was a massive fan of online sims, i've tried nearly all of them, from the console ones like GT and Forza to the PC sims like RFactor Live for speed and now IRacing and i have to say IRacing beats them all.
The only thing with IRacing is the cost, What you do is pay a membership which gives you a few tracks and cars. You then buy the cars and tracks separately, getting discounts for bulk buying. All the tracks are laser scanned and are as near to the real thing as you can get, every bump and camber is true to real life.
It started with mainly Nascar but has now incorporated a lot of other classes. There as several professional Nascar drivers that regurly use it for practice and fun, as the tracks are so good. There is F1 but only one car the 2009 williams, while it may not appeal to some the reason there is only one is license costs and the time it take sto get the phyics as near to real as possible.
If you have a decent wheel and a bit of free time i would strongly recommend checking it they usually have special offers of a 3 month membership for the price of one for newcomers.
If you love your sims check it out you will not be dissapointed