It's hard to really sum up Supergods as anything you try to identify it as leaves critical bits out. Yes, it's a pop history of superheroes written by someone from the inside and has a more immediately point of view than most superhero comic histories tend to be. Yes it's Grant Morrison's autobiography, more or less, and yes, there's a lot of Morrison's attitudes on superheroes, fans, and various bric-a-brac.
It's also a book wherein, blasted out of his mind of hallucinogens in Kathmandu, he is given the ability to see things in five dimensions and translate what these hyper intelligent pan-dimensional beings have told him. Also, apparently, one must cross dress and do magic effectively. The image of Morrison in latex stockings, a blonde wig, and fire-truck red lipstick (it doesn't say, but really, I'd like to think so) acting all "chaos magician" will sustain me during those times when I find Morrison a bit too pompous and difficult to take.
In short, it is exactly what Grant Morrison would write if Grant Morrison wrote a book, which means it's insightful, pompous, ridiculous, clever, engaging, and every other adjective that leans to mind when one thinks of Morrison's work. I personally enjoyed reading it for no other reason than you can almost smell his neurons burning--there's so much thought at work in its pages that even if you hate one tangent, there'll be another one along in a few seconds. It also explained why his New X-men run ended how and why it did, and confirmed a few things I'd always expected.
I suggest everyone give it a read. You'll find some good food for thought there.