LegalZoom worked fine for me when switching to an S Corp. I think I spent about $750 with them and it was quite painless. Previously I have gone the LLC and S corp route but using a local Attorney, cost me probably 3 times as much and I had to make several trips to their office. LegalZoom does thousands and thousands of these things and have it down right. It's really just filing paperwork and paying fees, you can even do it yourself if you don't mind frustration. Myself, it's cheaper and less stressful to pay someone who knows what they're doing.
Now myself, I don't see any real issue about who does the paperwork for you, long as they're a legitimate company. LegalZoom or any of the others should be just fine. BUT, just getting that status is the simple part, you must have a good Accountant to keep you current with all the rules. Especially important with either an S or C Corp but I think necessary as an LLC as well. You can get yourself some very major headaches real quick, simplest and safest to hire a Professional to maintain this essential part of your business. You have quarterly filings, State and Federal tax issues that are different from a sole proprietor, much more scrutiny from regulating agencies and who knows what else. So pay a good Accountant to sit down with him monthly and review your business and to keep you compliant. This sort of thing will save/make you more money than picking up a dozen residential accounts.