The biggest problem is what do they really want. Their letters are sketchy and they don't let you email them for further info. (At least not me). Also, they will not do document sharing such as Google docs or Drop box, so you need to put everything on disk.
The best course of action with them is that more is better and put it on disk and drop it off. They may call you in for a statement, at which time you may discover what they are really looking for. The statements are confidential and you don't get a transcript. But they are interesting.
Their mail is unreliable, their fax is worse (they turn it off if you are sending a long, fax, and they don't have efax or smartfax).
Always get a return receipt and produce it if they claim they did not get your info.
Unfortunately, dealing with the ARDC is about the same as dealing with your most onerous opposing counsel--and then some. If you have encountered dirty tricks, lying and cheating from opposing counsel, then you are ready for ARDC litigation counsel, sorry to say. I experienced obstruction of justice, witness tampering and spoliation of evidence, and if you complain, it will be ignored.
They won't be your friend. They may be helpful but don't count on it.
It is best to have an attorney at a statement or if the claim proceeds to litigation, but no one wants to go up against the ARDC. They have a "list" of 30 defense attorneys, but out of the 30, only 5 will respond and even those attorneys are reluctant to get involved.
The ARDC really should set up public defenders to assist and be there when you are called in for a statement and esp. when and if it goes to litigation. Indigent lawyers and pro bono lawyers should get a PD for free, and the others on a sliding scale based on recent income information.
Good luck to everyone and if you have questions you can email me at joanne@denisonlaw.com.

I could not agree more. I have had complaints leveled against me where if the ARDC had simply read the transcript which accompanied the complaint they would not have wasted their time bothering me. I also wonder just when it was that the ARDC sought to assert jurisdiction over complaints made by neighbors for conduct in YOUR OWN BACKYARD. I had a disagreement with an adjoining neighbor regarding a drainage issue. She took plenty of photographs and filed a complaint with the ARDC about my "unsavory: conduct. Rather than asking her why she was bothering ME in my own backyard, they turned to me and asked for an explanation about my conduct.
The conduct police are here folks... better get used to it. The ARDC is like any other institution out there.... they must justify their existence and their conduct even if it means forwarding the most ridiculous complaints and forcing us to waste our time responding. No thanks.....

Above all, avoid getting mad that the inquiry letter arrived. This is serious business and it is easy to fall into the trap of letting anger over what may be an unjustified client complaint lead to a poor response. Get a lawyer to help. A lawyer brings an outside view to the situation and can help figure out what the supposed rule violation could be and help address it objectively. We have good success downstate in this approach.
Also you may need to notify your E & O carrier. Your policy requires notice to them as soon as you are aware of a potential (not just actual) claim. Further both CNA and ISBA Mutual provide a fund for fees for the lawyer of your choice to defend the ARDC inquiry or complaint.
I am not sure about others on ARDC's list of lawyers doing ethics defense, but those of us downstate return calls to any potential client.

It is the ARDC's practice to ask for a response on nearly every complaint they receive, with the possible exception of some prisoner complaints, so don't take it personally if they ask you for a response to something that seems ludicrous. If, on it's face, the complaint could possibly constitute a violation of the Rules of Professional Conduct, you will be asked for a response. It is always a good idea to have someone else take a look at your response to the ARDC before sending it, whether it's a partner or hired counsel. If you have ANY concerns about the merits of the complaint, you should hire counsel at the earliest possible opportunity. All too often, attorneys don't hire counsel until after a formal complaint is filed, which might have been avoided had they hired counsel earlier in the process. Nearly all of the defense attorneys on the list the ARDC provides are either former ARDC prosecutors and/or experienced ARDC defense practitioners (this writer is both) and, while I can't speak for everyone on the list, most of us return phone calls promptly and are very responsive to our clients' needs. Dealing with the ARDC can be stressful, but it can be a great comfort to know that you have someone knowledgeable in your corner, especially if you think you might have some exposure. As noted above, be sure to check with your carrier as many policies include at least some coverage for ARDC defense fees.