It is as serious as sim racing gets, which doesn't necessarily make it better/more enjoyable (and people will argue about its realism all day) - only that you have to approach iRacing with a certain mindset. There's a driving line in the rookie series, and you are allowed shifting aids, but that's it. There is no AI - you must race against other people - and you are expected to be clean at all times. No contact, no cutting, fair racing - you will never make progress if you drive like it's a 'game'. Some people see that as nofunallowed.jpg (although there is a Test mode where you can muck about all you like), while others appreciate this approach and treat it like a real racing career/hobby, looking for the same type of 'fun' that you'd have in a sanctioned race series in reality.
It is less 'pick up and play' than GT - there is a calibration wizard to help you set up the controls if you want to jump in quickly, but that is just the beginning - there's a huge number of settings that you might want to test out to improve the feel to your tastes. In terms of physics the cars will initially seem unforgiving compared to GT - this is apparently down to the tyres, which are simulated in a unique way, and struggle over the limit, meaning slides are difficult to save once they are beyond a certain slip angle. The benefit of this tyre model is in the detail it offers over long runs - they evolve and behave differently depending on your driving style, how your car is set up, etc - but there is still a lot of room for improvement. I would say GT6's tyre improvements made a step towards iRacing's tyre characteristics of fast, unsavable grip loss at high slip angles, but they are still move forgiving in comparison.
In terms of how it feels with a T500, I'd say all PC sims with a T500 have the potential to feel better than the T500 on the PS3. Being an official wheel, GT6 is calibrated pretty nicely with it, but it comes alive on PC - it simply feels more responsive, and the driver options and individual sim options allow very fine tuning to your tastes. iRacing's FFB in particular is superb - linear mode is a revelation, even though the T500 isn't really powerful enough for it. I recommend seeking out the tips for FFB calibration in the iRacing forum if you're going to give it a try.
I think you can only answer that yourself. I would imagine it's a very difficult thing to do, particularly if you're fighting against the feedback. It will want to rotate its base while you're holding the wheel - you can't just rest it on your lap, you'd have to grab it between your thighs... which would then make pedal control kinda challenging too... if you put the strength to its lowest setting in the game you might be able to do it, but it's far from ideal. I think some very weak wheels came with lap clamps/supports for this purpose, but none of the powerful ones do because it would be wayy too uncomfortable.