Charles Leland's "Aradia" has definitely been an influence on modern Pagans, particularly witches and Wiccans. One of the primary Wiccan texts, "The Charge of the Goddess," is drawn in part from material in "Aradia."
There are also modern witches who model a lot of their practice on "Aradia." The works of Raven Grimassi, for instance, tend to draw a lot on "Aradia."
There is some scholarly doubt though about the authenticity of "Aradia." Some authorities believe that Leland made up most of the material himself. Others believe Leland honestly believed the material was all authentic, and was fooled by his informant, Maddalena, who he attributes with supplying the information that was used to create "Aradia." Perhaps Maddalena did make it up herself, or perhaps her unnamed sources made it up. Or perhaps the material is all completely authentic. At this point it's not conclusive one way or the other.
It is very likely that at least bits of the material in "Aradia" are genuine Italian folklore. Genuine folklore does not necessarily mean though that it was ever part of a self-professed witch's practice.
Regardless whether the material is authentic or not it is now used in a number of spiritual systems that do provide real benefits to those who follow them. As many witches say, "if it works, use it!"