I had a friend who did not use an attorney to file N-400. At the time, INS lost his fingerprint letter and sent him a letter after 6 months of filing that he abandoned his application. Next time, he used an attorney, all his notices and correspondance from INS came on time. Another, benefit of using an attorney is that the mailing notices go both to you and to attorney's office. So just incase one gets lost, there is a good chance that the other will make it. Also, in the worst case if both gets lost, an attorney can argue thru' proper channels to the right people within USCIS. My recommendation is to always use an attorney.
I have heard that attorney will handle N-400 preparation, review, follow up with USCIS, if needed, and intreview preparation for about $1000. BTW, you don't need to have a local attorney, any immigration attorney from out of town or state can handle it.