Even though some jurisdictions are careful to state that a mere breach of the code does not create a cause of action, the code nevertheless has been held almost universally to define the minimum duty owed by a lawyer to his or her client. While some duties are common to all lawyers, e.g., avoiding conflicts of interest, the nuances of meeting that responsibility to a corporate client can be substantially different than to a client in a child custody case, and what constitutes a breach of that duty can also differ significantly. Whether or not that duty has been breached can be a question of law or fact depending on the circumstances. Malpractice in appellate law, for instance, is an area a layperson has no business deciding, according to many jurisdictions, and is solely a matter of law. Legal malpractice cases can be nipped in the bud by a sympathetic jurist.
Expert testimony is, of course, generally required.
When it comes to expert witnesses, the greyer the hair, and the longer the track record in the same area of law, the better. Retired judges do nicely. A past grievance or malpractice case against your expert is not necessarily a fatal flaw. Mistakes corrected graciously and honestly go a long way toward making the defendant who is resisting doing the same look worse by comparison. The toughest element in a legal malpractice case, however, is proximate cause, since it requires proving that the client, but for the negligence of the lawyer, would have prevailed in the underlying lawsuit.