When I was earning my instrument rating and after I had become pretty cocky on my ability to fly the gauges, my instructor took me up into a layer of thick stratus. I'm hooded, climbing out and before we hit the clouds, my instructor said to take the hood off and look around. I did. Nice view. Then suddenly we're in a bowl of cotton candy. Old crappy trainer, the air vents start leaking water. My heart rate went up times two. I'll never forget that day and how true IMC felt the first time.
This October past I earned my CFI. It's not in the syllabus, but I would love to give my students a flight into real IMC. Of course they wouldn't pay for it.