I think it's kind of good only when nothing else works with the cell in concern and you only need a transient expression. For me with most primary cells I use lenti and retroviral transfection solves the purpose. Moreover if you can have your cells to produce a therupatic protein it's always better to have it express it permanently. I mean unless you choose something like neuron or another non dividing to produce it. Anyway, while we are at it anyone knows the best method for ssDNA transfection to work along with CAS9 gRNA lenti tranduction, for primary cells?