If you've already enjoy Elephant and Piggie and you're on the fence about this one in particular, hop off immediately and buy the book. Even if you have one or two of the stories, this format is great since it's compact and it's cheaper than buying the books separately.
If you haven't read Elephant and Piggie, you're in for a treat. Mo Willems is an amazing author and illustrator. My 6 year old loves these stories, and he WANTS to read them all the way through, unlike most other books. He usually reads Piggie and I read Gerald. The books are great for teaching how to read with emphasis (words in uppercase or italics) and emotion. The illustrations are a lot of fun and Mo Willems is a master of illustrating emotions on the characters. I have just as much fun reading the books as my son does, and that rubs off on him.
I bought all 3 of the "Biggie" books. So far we've read all of 1 and 2. IMO there are 1 or 2 that are better than the rest in each book, but all of them are entertaining. The Biggie books also have fun intros to each story, which we have a good time with. I could go on but you have better things to do, like buy and read this book.