This work is often discussed in terms of its value (or, rather, complete lack thereof) as history. I was interested in it as literature, as I knew that it introduces Lear and is likely the earliest text to connect Merlin and Arthur.
The Lear story is very well told here, and there are other vivid episodes such as the reconciliation of Belinus and Brennius by Conwenna (called Tonuuenna in Thorpe's translation), but the most remarkable part is Book Seven: the Prophetiæ Merlini, political prophecies weird and apocalyptic in tone.
Most of the rest of Geoffrey's Historia is given over to descriptions of battlefield slaughter, with much cleaving of heads and vomiting forth of blood and souls. Very good stuff!