This isn't entirely germane to the question of "what is the difference?". However, I am about halfway through my IMC training. One thing is clear to me: I am a better pilot now than I was when I started. I am also convinced that the improvement in my skills, attitude and professionalism is due in large measure to the extra discipline that is imposed on me by having to aviate, navigate and communicate without visual reference to the outside world during my training. This improvement has been observed not only by myself but also by people who fly with me regularly who have commented on the greater precision with which I now fly in VMC.
However, what is also clear to me is that while the toolkit of skills I am learning for IMC is "good enough" to get me back on the ground safely, it is not going to be refined to the level where I would have comfort departing in IR minimum conditions, flying cross country in those conditions and landing on the other end in those same minimum conditions. In my mind, that is the practical difference between the IMC and the IR.
Re: the amateur vs. professional debate. I have always stuck to the basic premise: "Amateurs practice until they can get it right, Professionals practice until they can't get it wrong"