As a treatise on the history of superhero comics, this has the virtue of brevity. I'm not well-versed in the distinctive qualities of the golden-silver-copper ages of comics, only really being interested in stuff that came out in the 90s and 00s. Flex Mentallo does provide some clues as to the different properties and enjoyments of each age, tho in general the narrative links up all four issues pretty tightly so the distinctions aren't especially clear (the introduction does help a bit in getting more of the flavour of each age).
While the comic's formal and thematic content is pretty interesting, the central character isn't especially accessible – perhaps because unlike Grant Morrison, superhero comics do not have the ability to fundamentally alter my perception of reality. Wally's suicide attempt is due to a fatigue at how bleak and depressing comics (and by extention the world) has become. Oh, and nagging from his girlfriend – not v well explained, that. All of which makes me think that All-Star Superman is a more affecting statement of Morrison's case.
What's most interesting in Flex Mentallo is the villain – the 16 year old teen dismissing superheroes as "pathetic fucking power fantasies for lonely wankers". The source of this ire is actually self-hatred and envy at the guys with girlfriends and awesome life experiences. What's missing is Morrison's belief that ideas are there to be realised and superheroes are there to be emulated. You can become that power-fantasy, basically, if you will hard enough (and have the appropriate sigils).
One little craft effect I liked is the way one panel would bleed across the whole page, with the other panels being laid over it – quite a nice way of emphasising how the different strands of the story intersect together. Also the great opening sequence across the first two pages – muddling up the creation of the universe and the creation of the comics page while also linking in with the later symbol of ideas as eggs. The comic ends on a splash of a kind of apocalypse-turned-rapture, with superheroes finally coming down from heaven to save us from our imperfections and construct utopia. It's a superhero holy text, starting with Genesis and ending with the Book of Revelations.