If you are or have ever been in immigration court deportation (removal) proceedings, run, don't walk to see a lawyer. If the proceedings are not yet finished or are on appeal, your entire immigration situation is in the power of the courts—and you will not get anywhere trying to use the same application procedures as people who are not in proceedings. Even if the proceedings are over, you should ask a lawyer whether the outcome affects your current application.
The most common legal issue encountered by would-be immigrants is the claim by USCIS or the consulate that they are inadmissible for one or more of the reasons listed in the article, Inadmissibility: When the U.S. Can Keep You Out. Possible examples include having committed a crime or previously lied to the U.S. government. If you know that any of these grounds apply to you, it makes sense to get legal help before you begin the application process.
Even the simplest of U.S. immigration applications involves filling out some forms, and you will most likely be asked to follow some detailed instructions about gathering and including other paperwork and fees. Make a mistake, and you may find your application returned, delayed, or even rejected.
Immigration lawyers have dealt with this paperwork countless times before you, and have both the knowledge and the streamlined systems to prepare the applications smoothly. They have computer programs in which they can enter your information and spit out the forms in an instant. Hiring a lawyer can be well worth it for the peace of mind alone.
Another circumstance that often drives people to lawyers is the failure of USCIS or the consulate to act on or approve the application, for reasons that have more to do with bureaucracy than law. For example, an applicant who moves from Los Angeles to San Francisco after filing the green card application might find that the application, which should be transferred to the San Francisco USCIS office, has instead disappeared into a bureaucratic black hole for months. Delays at the USCIS Service Centers are also far too common.
Lawyers do not have a lot of power in such circumstances. But at least the lawyer may have access to inside fax or email inquiry lines, where they (and only they) can ask about delayed or problematic cases. Unfortunately, even lawyers have trouble getting answers to such inquiries, but it’s often worth a try.
Whatever you do, don’t rely on advice by USCIS information officers. Seriously. Would you want the receptionist in your doctor’s office to tell you whether to get brain surgery? Asking USCIS information officers for advice about your case (beyond basic procedural advice such as where to file an application and what the fees are) is equally unsafe. The people who staff USCIS phone and information services are not experts. USCIS takes no responsibility if their advice is wrong—and won’t treat your application with any more sympathy. Even following the advice of officials higher up in the agency may not be safe. Always get a second, preferably lawyer's opinion.