Depends on the gene defect; if it's enough to edit 3% of your liver cells to produce an important protein to circulate in your blood, then it's pretty reasonable to expect to cure that disease with (non-germline) CRISPR/Cas.

If it's something that needs to be fixed in 100% of a certain cell type, especially non-proliferating cells, then that's going to be very tough. And if it's something that acts during development, then fixing the DNA in an adult would do nothing (the body has already been "built" with the wrong "blueprint").