Like a House on Fire

I'd never heard the phrase "they get on like a house on fire" before, and now I've heard it twice in two days. The first time was last night in a Bill Bryson book, and the second time was this morning in a Daily Mail article about Sarkozy's new girlfriend. After confirming with friends that I'm not simply ignorant for being unclear on its meaning, I googled the phrase and found the etymology. It essentially means that people get along really well, though it can also mean that people take to each other quickly (which was my supposition of what the term meant).