It's good to see that this kind of research is being done, but having read the abstract of the article I can't help but feel a little anxious.
This is a treatment against leukemia, but previous gene therapy research (about CD34 instead of CD19) has shown that modifying cells in the immune system (or associated with them) with retroviral transduction techniques can in turn cause leukemia itself. The goals of these two studies are very different but the potential side effects are very similar.
Now the enormous difference is that gene therapy is in vivo transduction and that the transduction in this study was done in vitro but was performed, like gene therapy, using retroviral transduction. Retroviral transduction has the potential downside of incorporating genetic material into unfortunate places like protooncogenic promotors/genes, which could promote unwanted (over)expression of said protooncogenes.