Much of my recent research has been in 17th century probates in Suffolk, Essex and Middlesex Counties, Massachusetts, as I am filling in gaps, loose ends, and busting one brick wall (which of three women of the same name, all born within 3 years, was the one who married my 7th g-g-f, which involved tracking down every available probate for their grandparents, parents, stepparents, uncles, and siblings). Ancestry's database of the Essex probates is a complete mess, but thankfully the NEHGS, in cooperation with the Massachusetts Archives, has them fully indexed, excellently organized, and free.
The difficulty is that black-and-white digital reproductions can sometimes be difficult to read, and any digitization should be full-colour. I have some copies from the Principal Probate Registry, obtained back when they were £5 each (with the exchange rate and the cost of a money order and postage, I think they cost me about CAD12 at the time) and they are very faint.
The other valuable part of the probate file has been the "miscellaneous papers". The bonds of executors and administrators have given clues to family relationships (who signed as surety?), and bonds of guardians have sometimes mentioned the age of the children. Other documents have revealed disputes between the widow and children of the first wife - very useful when birth or marriage records are missing and allocation of the children among multiple wives is difficult. In an intestate estate, the administrator's account is usually the only mention of the heirs. In my very limited experience of the British system, I have not received copies of anything other than the will or the grant of administration, so I don't know what other papers British probate files might contain.
My best discovery, however, predated the digital age. When I first started out in the 1990s, I was living in the Boston area, and did research in person at the Massachusetts Archives. They had, in the reading room, the LDS microfilms of the register books and indices, as the primary resource. (In the American system, the wills, inventories, and orders of the court are transcribed into bound volumes). So I was relying on transcriptions, and in some cases transcriptions of transcriptions, as some books had been copied in the late 18th or early 19th century. In the index entry for one of my ancestors, the immigrant Joseph Jewett, instead of any book and page references, was only the notation "see file". I inquired at the desk, and they retrieved for me the original file from 1661. It was a magical experience to hold the paper and to see the actual signature of an ancestor. Every published genealogy gives Joseph's date of death as 26 February 1660, which, according to the published records of the town of Rowley, was the date of his burial. The heading of the inventory, however, stated "The inventory of the estate of Joseph Jewett, who dyed on the 24th day of February 1660". I had the opportunity to handle 2 or 3 other wills that predated the use of the register books, but that was the most valuable find.