Turns out, not very:
A huge commercial success from the day it hit bookstands, Travels With Charley in Search of America was touted and marketed as the true account of Steinbeck’s solo journey. It stayed on the New York Times nonfiction bestseller list for a year, and its commercial and cultural tail—like those of Steinbeck’s Of Mice and Men and The Grapes of Wrath—has been long and fat. For five decades Steinbeck scholars and others who should know better have not questioned the book’s honesty. But I had come to realize that the iconic American road book was not only heavily fictionalized; it was something of a fraud.
I'd say "fraud" is too harsh a word but when you're dealing with writers who make up stuff for a living, you have to expect that the work they insist is true may not be entirely so.
I'm not disappointed to learn this. I didn't expect the book to be straight journalism - no one reading it can come away quite believing everything Steinbeck has to say. Bill Steigerwald in the above linked article as well as his blog entries has done a fine piece of literary excavation and found a truer version of Steinbeck's story. And he also found there are plenty of people out there who, well, can't handle the truth. Steinbeck might've found that amusing and worth writing about, too.
I think Steigerwald's work sheds a different light on Steinbeck's book but says more about Steinbeck's supporters and promoters than the man himself.