it depends alot on the type of marijuana..
pure sativas, as far as i can see, have totally smokable leaves. with even a pleasant taste, you just need more bowls to get high,though the high is not as intense as the flowers.
even the fanleaves are smokable.
in other types of mj, well, as far as i can see, indica leaves are totally unsmokable unless its the sugarcoated leaves and even those will be harsh. fanleaves might give ya a headache. smaller leaves without sugar, might get ya high, but are Really harsh tasting (unless months old cured..and even then, not that good)
might be mostly because of the higher content of chlorophyll (green)
hybrids containing more sativa are usually more smokable, though i havent found one yet with fanleaves that were smokable.
now when i say, smokable, i mean they get ya high, though to less extent than buds (sometimes much less)