I personally find When() and Decode() easier to read in these cases.
So this:
IIF($feature.field == 'apple','fruit',
IIF($feature.field == 'carrot','ewwwwww',
IIF($feature.field == 'candy','yummy','')))
becomes...
Decode( $feature.field,
'apple','fruit',
'carrot','ewwwwww',
'candy','yummy',
'none')
and this:
iif($feature.Class_2012 <=9 , 1,
iif($feature.Class_2012 <=19, 2,
iif($feature.Class_2012 <=29, 3,
iif($feature.Class_2012 == 30, 4,
iif($feature.Class_2012 == 33, 5,
iif($feature.Class_2012 <=41, 6,
iif($feature.Class_2012 <=44, 7,
iif($feature.Class_2012 <=46, 8,
iif($feature.Class_2012 == 47, 9,
iif($feature.Class_2012 <=59, 10,
iif($feature.Class_2012 >= 60, 11,0)))))))))))
becomes:
When( $feature.Class_2012 <=9, 1,
$feature.Class_2012 <=19, 2,
$feature.Class_2012 <=29, 3,
$feature.Class_2012 == 30, 4,
$feature.Class_2012 == 33, 5,
$feature.Class_2012 <=41, 6,
$feature.Class_2012 <=44, 7,
$feature.Class_2012 <=46, 8,
$feature.Class_2012 == 47, 9,
$feature.Class_2012 <=59, 10,
$feature.Class_2012 >= 60, 11,
0 )
That way you don't have to count a bunch of closing parentheses.