I finished the Anne of Green Gables prequel. I thought the author did a good job of imitating Anne's voice, tastes and character. It reminded me of Syrie James' book, The Lost Memoirs of Jane Austen, in that regard -- it didn't sound exactly like the original author, but close enough that you didn't mind.
The only thing that bothered me about the book was that it was DEPRESSING. Not consistently so, but by page 350 I was thinking, "Please, let something good happen. Please." One of my favorite things about the series was that it was so positive, and any setbacks that did occur were mild -- Marilla giving Anne plain dresses, etc. At first, Before Green Gables had that cozy, escapist feeling, too; but after a while, to borrow Anne's words, it was "mired in the depths of despair."
Fortunately, it had a happy ending. I don't think I'm giving anything away (but stop reading if you're worried) when I say that it ends with her heading toward Green Gables. I mean, the book is called Before Green Gables, and everyone knows what happens next. I thought the ending was really well-done, getting into Anne's head and showing how she was leaving all her past experiences, good and bad, "on the other side of the Northumberland Strait."
This would explain why the characters in this book aren't really referenced in Anne's subsequent life -- an omission that bothered some reviewers. It made sense to me, and helped explain how Anne got over her early tragedies.
In summary, I would say if you're worried about inconsistent tone or bad writing, there's no cause for concern. But if it would really bother you to see Anne undergoing some fairly extreme hardship (albeit with a happy ending), that might be an issue. I don't regret reading it, but I think some of the events could have been softened a little.