I guess he answer might depend on your goals. Are you trying to produce a recombinant protein or are you trying to make a virus. I have used Cellfectin II many times with a viral vector that is about 120 Kb in size with good success. In my case even low level transfection is okay since infected cells produced virus which infected other cells. If you are working with large sized DNA be sure to treat it very carefully, avoid freeze thaws and don't vortex or shear it. I hope this helps.