I've been reading a lot of contradictory accounts of the Ismailis, in Montefiore's "Jerusalem: a Biography," on their own official website, on Wikipedia and in various other places. Some sources present the Ismailis as an extremist splitoff sect from the Twelver Shi'as, while others portray them as a reasonable sort of protest group who've endured centuries of persecution.
Does either of these portrayals fit more closely with the historical records? Do they each have their own validity?
I'm not an expert on this topic particularly, but I would imagine the answer to this is complicated, because it deals with the perceptions and image of a group of people, something that has changed and is always in the process of being altered (often by the Ismaili community themselves) to fit political and social realities of the time. While doctrinally the Ismailis do represent an 'extreme' branch of Sh'ia Islam, this is in the sense that, say, Unitarians or Mormons represent an 'extreme' version of Protestant Christianity - it more represents divergence from common theological views than any sort of 'extremist' practice. The Ismailis themselves consider Ismailism to simply be a particular practice within Shi'ism and following a commonly accepted school of Shia fiqh. Certainly the image that is presented by Ismailis nowadays is that of a progressive, intellectually and culturally productive branch of Islam that has suffered persecution throughout the ages, at least until the eighteenth century when they gained some protection in Qajar Iran and the nineteenth century in British India. In a certain sense the Ismaili historical narrative here borrows much from the Jewish experience. Yet in diasporic communities, where Ismailis may actually represent a significant fraction of Muslims in a city (for instance, in Toronto, where I am from), the Ismaili community instead promotes an image of Ismailism as the progressive face of Islam in general, and the narrative that the Ismailis are a persecuted minority descended from an 'extremist sect' is often understandably downplayed. So essentially, both of the portrayals you describe are 'true', but it depends which narrative you are inclined to follow. You may be interested in the book 'The Ismailis in the Middle Ages: A History of Survival, a Search for Salvation' by Shafique Virani, which is (as you can tell from the title) obviously telling the persecuted minority narrative, but is written very clearly and gives a good example of how the historical record is made to fit a certain view of Ismaili history.