So to the writing. I grew up with Monty Python, and can contribute worthily to any (mercifully rare) Python recital (Obligatory xkcd). I was open to the idea of the material being worked into a musical, and interested to see how the writer integrated the recycled/reworked material into the show. I'd listened to the soundtrack and knew roughly what to expect, and was content to compensate for the smaller production.
The show is basically chunks of Holy Grail floating in a puddle of Broadway parody and self-parody, with only token attempts made to integrate the two. For the former, I had a very similar sense to when I saw Tomfoolery done by Kit & the Widow - excellent and well-loved material done slightly less well than the original, with occasional sparks of the performers finding a new effect within it. For the latter, Forbidden Broadway have already torn The Song That Goes Like This to shreds (Spotify link), and I saw little originality in the other numbers. True, it is the nature of parody to imitate, but Diva's Lament was done in 1896 by Gilbert & Sullivan (The Grand Duke), and it was just as incongruous and ineffective then. You won't succeed in showbiz (a UK rework of You won't succeed on Broadway [if you don't have any Jews]) was funny, but it was little more than a string of cheap X-Factor references, leaving rather a sense of playing to the lowest common denominator.
There was little in the way of plot or character development (admittedly true also of the film). A few attempts at pathos are stitched into Act 2 (I'm all alone, and the inevitable Always Look on the Bright Side), but could have been cut altogether without affecting the "journey" of the characters, for the simple reason that they were the journey. Nothing happened outside the numbers, worthy though they were, to suggest that they fitted into the show.
So what are left with? Some funny and silly sketches (mostly familiar to most of the audience) knitted together with big-spectacle production numbers, lowbrow topical gags, camp humour, explicit crossing of the fourth wall, a cringeworthy scene of an audience member dragged on stage, scripted corpsing, and a sing-a-long at the end. Spamalot is a pantomime. An enjoyable one, to be sure, but little more sophisticated than the average Puss in Boots. It sends itself up without ever parodying anything which hasn't been done a dozen times before. Watching it was like eating a fondant fancy; sweet, colourful, and ultimately unsatisfying.