In my limited experience so far with the trial, it does do away with a LOT of tweaking, and I mean about 98%. You still want to use EQ, and you still want to tweak MIR itself, moving instruments etc, and like the previous post just said, using different instances / rooms for certain instruments to achieve a desired effect. Like the manual says it doesn't matter what the stage layout looks like, but how it sounds, so if you want to place a certain instrument in the far rear corner because it sounds right even though you would never do that in real life, go ahead and do it. I still use some eq, but only very simply, to shape the sound, and find that yes, MIR so far, is definitely doing 98% of the hard work.
Of course you can get into all kinds of creative engineering with MIR, like an engineer would when recording from a real room. But the point is that for most people it was simply a task to use EQ, compression etc, as a way to try to fake the realism - so now you don't have to do that - it's automatically (more or less) there. If you want to get into more creative things, that's another story. For many composers I think a set and forget approach is now within reach - concentrate on writing and programming the parts. For others who want to take it to the next level, you can absolutely do that too.
The funny thing is that the best fun I have had with MIR so far was with a loop of a shaker, haha, and I used the audio input feature to send it to a spot on the stage, and it just made it work.