Today, many Muslims seem to take the major narrators such as Sahih al-Bukhari and Sahih Muslim as pretty much fact, as well as the Sunan narrations. From a historian's perspective, is there any reliability to these hadith? Are they simply folk tales with little basis, are they a questionable mix of real stories with later inventions, or truly reliable?
The level of reliability (if any) that can be placed on the hadith is one of the most intensely debated questions in the historiography of Islam. I've written about some of the problems in an old post here: https://www.reddit.com/r/AskHistorians/comments/5z2suc/how_accurate_is_the_sirah_mohammads_biography/dewoqrw/
The general consensus is that they certainly can be said to reliably represent what 9th century Islamic legal scholars thought, so in that sense they have some definitive use.
Beyond that it's difficult to say. Some scholars still maintain that they have some "kernel of truth" and can be usefully read to find out true information about Muhammad while others think that for that purpose they are effectively junk.