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I agree with The Wise Asp about the first book being close to the original, but over the ~3 years the series has run its course, it's been getting a lot more creative. I only recently found it and read it all in a day, so it was easy for me to see the author getting better with each book. Fourth book promises to diverge heavily, with the introduction of Opal Koboi, Voldemort finally getting a body, and the Horcruxes.
That being said, the first few books do have a few odd details that I think are important. Already some of them are coming back in a very significant manner, judging from the latest chapter.
The story remains pretty much the same as the original PS, which is usually a sign your crossover idea doesn't really work. I'd wait for the sequels to be finished and then give it a try. It's a good read, don't take me wrong, but it's nothing outstanding.
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