For me, modeling heaven is that first base coat of paint. You have finished the construction, primed and preshaded, prepared your paint mix and then sprayed. At last you see your model looking something like you envision it. To me that first base coat is even better than than putting it down and saying it is finished. Maybe it is the contrast between the rough and ready primer and the smooth base coat but it always makes me realise how much I enjoy our hobby, trouble is I so often start anotherkit and put of second coat, waethering etc.
Hell, canopies, painting road wheels, fitting fusalage halves together.
What about you, what are your modeling heaven and hell moments?