Decent film; way different than the book. (spoilers)
One huge difference right off the bat is that Ethan died in the first book ( A Dog's Purpose) while hallucinating that Buddy is Bailey. Ethan goes on to be a supporting character in this film.
The book spans about 80 years or so with Gloria (CJ's mom) being much more of a monster her entire life until her death. There is never any sort of reconciliation between Gloria and anyone else.
When the dog dies for the final time, he wakes up in the water then swims to shore to see all of the people he loved in his previous lives.