I have tried it a bit and enjoyed it, but haven't taken the full plunge. My biggest reservation is that unless you spend serious coin you don't get any force feedback other than in the wheel. From my time on track it feels like my "butt sensor" is almost as important as my eyes. I can feel the car is going over the limit before I can see it's going over the limit. From what I've heard from others, SIM racing seems to be a great way to learn a new track, refine your line, and possibly even learn some wheel-to-wheel dynamics (assuming you have a 3 screen or VR setup). But it doesn't seem like a great way to learn the advanced car control that would get me a few more seconds. But I could be wrong! I probably need to try out some higher-end setups than the single-screen, G-27 rig I have.
FFB will never be truly accurate compared to real life. I wouldn't worry about that part too much, I actually had mine set quite light most of the time. Also, don't underestimate the single screen/G27 setup, that's what got me to Silverstone for GT Academy and taught me about vehicle dynamics/slip angle/etc. I didn't get started in the 'real world' until I had already accumulated nearly a decade (and 10,000+ hours) of sim racing and already had the muscle memory to catch slides without it being a conscious process, it just happened. My first unrecoverable slide actually just happened a couple of weeks ago at MSR-H over the launch, woops. So there's more that carries over than simply learning lines, but it may take more time investment than most people are willing to put in. With sims you have the time to really learn the nuances of what separates a quick lap from a FAST lap. That's my opinion at least. It's worth noting the last two winners of the Mazda Shootout were blazing fast sim racers. One was in GT Academy 2011, while the most recent was at Silverstone with me in 2012. We're taking over! :p
Sim racing is also a good place to dig in and learn how to analyze data. I barely scratched the surface, but you can jump in head first and learn how to analyze data with something like MoTec.