I'm a bit surprised nobody's dealt with this yet. To put it briefly, Mommsen is immensely influential. While he got a lot of stuff horrifically wrong, and often got stuff right for the wrong reasons, Mommsen's influence is not so much in what he thought was true, but in the methodology and place of ancient, particularly Roman, history. Prior to Mommsen there wasn't really any such thing as ancient history or ancient historiography, in any kind of rigorous, disciplined way. You had people like Gibbon: educated and well-read interpreters of Suetonius and Plutarch and all the ancient historians who nevertheless did not operate according to a systematic method and who rarely if ever practiced any kind of source criticism of any kind. By contrast, the way Roman historiography is done now would be very much recognizable to Mommsen--the material used (especially archaeological) and the questions examined would be very different, but the actual method is pretty much the same as that that Mommsen more or less invented. Apart from establishing a systematic methodology for the study of ancient history (typified by not only his unfinished history of Rome but also by his several thousand papers and other scholarly volumes) Mommsen was enormously influential in expanding the material that historians looked at. Mommsen published the codex Theodosianus and the Digest--the systematic study of Roman law is more or less founded by Mommsen, who apart from this wrote two books on Roman civil and criminal law--which became of use to historical studies. He published the Corpus Inscriptionum Latinarum, arguably his most enduring legacy to the modern scholar of Roman history, the first complete collection of all Latin inscriptions known at the time (it is still regularly added to and is a fantastic resource), as well as more or less introducing into the field of ancient history the use of inscriptions and monuments as historical sources. For that alone he might be worthy of tremendous praise, since the study of epigraphy, now an important part of the ancient historian's toolkit, did not really exist before Mommsen, who pioneered the study. And even if Mommsen's magnum opus, his history of Rome, holds little value for academics except as a piece of foundational historiography (as a piece of history it's utterly obsolete), the traces of Mommsen's history can be easily seen throughout our culture, to the extent that most popular histories are still basically just rewrites of Mommsen with a little extra contemporary color thrown in. This entire strain of thought that erroneously attempts to equate the contemporary world with the Roman world could possibly be said to have originated in its current form with Gibbon, but really owes everything to Mommsen. Even a generation later commentators were criticizing Mommsen's grand history volumes for clumsily trying to equate ancient figures and events with those of his own time. This nonsense about the Gracchi being proto-socialists? That's Mommsen, and was identified by commentators in his own time. Caesar as either champion of the people or tyrannical right-winger (both decidedly unsupported by the texts)? Also Mommsen. Considering that Mommsen's grand history never entered into the Principate I don't think it would be too much of a stretch to say that what Gibbon is to pop history of the Empire, so is Mommsen (albeit a vastly superior scholar to Gibbon) to pop history of the Republic. Which does have some impact, if not much academically