The novel has two endings, the original one that doesn't end with the reunion of Pip and Estella, and a second ending that implies Pip and Estella will always be together. In the first ending, Estella marries a doctor, and she and Pip don't ever get together. Readers were upset with this ending because they felt Estella and Pip deserved to be together after they had both changed so much. Some critics feel the original ending is better because it's more realistic.
At this point, it's really up to you to choose which ending you feel is better and why. I would look at how Pip changes and resolves his conflicts in the book. By the end, he realizes that wealth doesn't insure happiness and that life is a series of events that shape us into the person we become. Pip returns to the values he learned as a child after struggling with the conflicts he encountered in London when he turned away from those values. Estella changes also, becoming a gentler, kinder person from the abuse she went through.
Now, ask yourself which is the better ending and how you can justify that it is better. Is it unrealistic to have Pip and Estella live happily ever after? Why/why not? Or, do the two deserve to have some happiness now that they have learned valuable life lessons? This type of question is a matter of personal opinion, and you should have enough justification for either one.