It's hilarious and absurd. It's also a genuinely beautiful film. The Pythons and their crew filmed (in uncomfortable conditions) in Scotland. It was difficult, but about halfway through they staged a screening of the footage they'd gotten by then, and they realized they were making something special. Good motivation for the rest of the (still uncomfortable) shoot.
The very first draft of the script was a hodgepodge of scenes that didn't really connect with each other, going back and forth between medieval times and modern times, with a climax set in the present-day where King Arthur and the Knights steal the Holy Grail from the "Grail Hall" in Harrod's, since Harrod's, the large, longtime store in London, has everything. Even with what I thought was an inspired gag (they use a getaway car driven by God; you know it's God because shafts of light radiate out through the windows), they junked almost all of that — repurposing the Harrod's scenes for the "Michael Ellis" episode of their TV show — to focus on the medieval (with one big exception, of course).
It's still a sketch comedy film, and only slightly has a plot, but Monty Python and the Holy Grail did this with so much style. It makes good virtue of its incredibly limited budget. Replacing horses with people clomping together halves of coconuts was a genius move they made in the script stage. (They used a horse for exactly one scene where a horse was absolutely needed for the gag, and that was it.) I like that the recurring "saying '5' instead of '3'" gag happened because they rewrote a scene while filming, to eliminate two actors from a scene. (The Killer Rabbit killed five knights as written, but they cut that down to three, and wrote in the new dialogue by hand, then found other places where they could do the "5 instead of 3" gag to make it a running joke. Add to the joke, as improv troupes try to do! "Yes, and...")
And Terry Gilliam's animation, and the film's final animation gag, and Graham Chapman being a hell of a leading man (and still funny) as Arthur, and its meta, calling-attention-to-itself quality ("Look! There's the old man from Scene 24!"), and the funny-sexy Castle Anthrax scene, and the absurdity of the Black Knight scene, and how it uses mostly library music (not all that original as music, but energetic) as a tool to undercut plot points and to add jokes, and heck, how deep with jokes even the soundtrack LP they released for the film was: Monty Python and the Holy Grail is something special.
(Did England 932 A.D. have TED Talks? It just did!)