Vonnegut claims that "God Bless You, Mr. Rosewater" is a novel about people whose leading character is money. The protagonist, Eliot Rosewater, has inherited millions of dollars (earned from robber baron ancestors) in a charitable foundation used as a tax shelter. He, however, uses it as an actual charitable foundation. His father thinks he's crazy for giving away money; a lawyer and a cousin want to prove he's crazy to get his money; his wife doesn't think he's crazy, but can't stand it in any case.
However, while money does feature prominently, it would be a mistake to think that the novel is about money. A recent Goodreads review describes it in terms of Occupy Wall Street: "If you belong to the one-percent economic bracket, prepare to be mocked by Kurt Vonnegut. If you are a 99-percenter, prepare to realize that the joke’s still on you." However, money is just the flavoring, not the dish itself. The rich folks range from charitable to greedy and industrious to lazy and ignorant, and being charitable is the experiment of someone who is far from normal.
I see this, like many of Vonnegut's works, as a humanist novel. The book is about loving people even if they're useless, annoying, greedy, or ignorant. Even if they hate you. Even if they say "God bless you, Mr. Rosewater" ironically egotistically. His baptism was also great: "Hello, babies. Welcome to Earth. It's hot in the summer and cold in the winter. It's round and wet and crowded. At the outside, babies, you've got about a hundred years here. There's only one rule that I know of, babies -- God damn it, you've got to be kind."
Some people criticized the ending, but I liked it. I can understand the criticism -- it was abrupt. However, it was also fitting. It felt right for the characters, right for the story, and right for the message. Also, the musical rendition of the ending was pretty fun (don't listen to it if you don't want a spoiler).
God bless you, Mr. Rosewater.