This is an oratorio first, and a comedy second. It's not your conventional Monty Python fare, nor is it a "Hollywood Bowl"-style celebration of Python humour.
What it IS is a tongue-in-cheek yet often straight-faced musical parody of Handel's "Messiah", with over 100 voices and a 120-piece orchestra, that has a sense of humour and is inspired by- rather than being a retelling of the story of- "Life Of Brian". If you know what you're expecting, and you're not expecting a classic 40th anniversary Monty Python reunion, then you'll probably enjoy this. Apart from an encore of "The Lumberjack Song" and the inevitable "Always Look On The Bright Side Of Life" this is not "Monty Python Sings".
It's not Eric Idle's finest work- I'm afraid I think "The Rutles" was the last time he was actually funny- and some of the new musical jokes are on the weak and obvious side, but the four lead vocalist's performances are vocally strong as well as being full of fun. It's enjoyable but not laugh-out-loud funny.
The Blu-ray is reasonable, with two behind-the-scenes documentaries (running for about 40 minutes in total) and some other extra pieces which help explain why "Not The Messiah" even exists, which seems more like a happy accident than a deliberate celebration or a cash-in.
This is only for real Python completists or for opera fans with a sense of humour, rather than a must-have for everybody.