I went through the whole process so give some of my opinion based on my experience. For NIW, law firm can't help you much, since all requirements are reference letters. Lawyer knows nothing about your field. He/she only submit your materials you collect. But application submitted from a law firm would look better than the application filed by yourself. Since Immigration office only spends about very short time (15-20 minutes?) to go through your application, every positive imprression counts.
My advice: Find a less expensive law firm without bad reputation. Effiency also matters now due to 10/1 deadline.
Hope it helps. Good luck!