This book was published in several parts, and it shows. The first segment, a fairy tale about Merlin's education of young Arthur and his eventual coronation as king, is lighter than the rest, both in tone and in prose. The second "book" (there are four), shifts into seriousness so suddenly that it's almost jarring, although the story of King Pellinore, which is woven throughout, is more like the first. Then we get to the third book, the longest of the four by far and almost entirely about Lancelot, to the point that I would argue that he's the main character of book as a whole, even more so than Arthur. All the shifts in tone and in viewpoint give the book a somewhat jerky feel -- I was never really able to lose myself in it. This was exacerbated by T.H. White use of Arthur and his time as a metaphor for World War II (the first three books were published in 1939-40; the last didn't appear until 1958). A good percentage of the book is given over to discussions of war, violence, questions of right and wrong, and the use of force. There's a whole chapter in the second book that's basically a socratic dialogue among Arthur, Merlin, and Sir Kay about the question of just war. But it's not just the characters who have these discussions; White inserts them in the narrator's voice throughout, even in the lighter segments of the first book. (One of the most memorable: a discussion of feudalism, and why that system really wasn't so bad. No, seriously.) Between all the philosophical ramblings and the long segments of story told via indirect narration, the book reads more like a textbook than a fantasy story.
Another thing that bothered me throughout the book was the timeline. Most writers set stories about King Arthur at around the time the Romans left Great Britain. But White pulls everything forward by several centuries, setting Uther Pendragon's reign during or around the time of William the Conquerer. (The timeline is weird, though, because Merlin refers to Uther as "The Conquerer" more than once, implying that Uther led the Norman invasion, but in later segments, William the Conquerer is also brought up.) Robin Hood is a character in the first book, and the resulting mish-mash of English legend didn't sit well with me at all. Otherwise, the story follows what I know of Arthur's story pretty closely. White assumes that his readers have also read Malory (which also adds to the textbook feel), so he skims over quite a lot. As it happens, I haven't read Le Morte d'Arthur but I know enough of the basics from reading other books that I was always able to follow along.
So, not my favorite book, but now at least I can say I've read it. If you're looking for a good Arthurian yarn, I'd recommend the Mary Stewart version, or The Mists of Avalon (even if MZB's "Christianity bad, paganism good" refrain gets heavy-handed), or my favorite retelling by far, Queen of Camelot by Nancy McKenzie.