I bought this for my (then 7 year old) brother taking him to see Digimon: The Movie in theatres. He enjoyed all things Digimon, but disliked all things book. The two cancelled out into a pretty neutral opinion.
Today, Charlotte read the book with me for the first time. To the best of my knowledge, this was her first exposure to Digimon. She's seen a couple episodes of Pokémon, and easily made the connection between the two.
She enjoyed the illustrations and the simple introductory story quite a bit.
As an adult, the story is an easy read. It's short, with only one or two sentences per page. The only drawback is if you haven't seen the show before (or have a few years of kid Digi-talk under your belt already) you'll have major problems pronouncing the monsters' names. Agumon, Gomamon, Piyomon and the gang have some pretty tongue twistery names if you're reading them cold.
Nostalgia plays pretty heavily here. If you don't have an interest in Digimon, I'd avoid the book, series and everything that goes with it. You'll save your self the headachemon it will inducemon. If your kid ends up liking it, you're opening a can of worms trying to track down books, shows and toys that are out of print and becoming highly collectible.
However, if you--the Parentmon, Auntorunclemon, or Guardianmon--loved Digimon, and happen upon this book in a thrift shop, it seems like a great introduction for a book loving member of the new generation.