i recently bought this series to add to my ever expanding blu-ray collection, i saw the series when it was broadcast and loved it. it is by far the best piece of nature television ever filmed and by that alone would justify your purchase of it. but unfortunately the picture quality isn't quite as good as i was expecting.
i watch my blu-rays on my PS3, hocked up to my 40" Sony bravia with a full 1080p resolution, and also connected to my Sony 5.1 surround sound, now apparently Planet earth was the first documentary to be filmed almost entirely in HD, i would disagree. within the first 5 episodes (these are the ones i have got round to watching on blu-ray) i would say only about 30mins of footage appears to be shot in full HD with these scenes looking crystal clear. but a lot of the time the image is very grainy and that presents a lack of detail almost as if you are watching the upconverted DVD version..
Now my advice would be, the hour or so of actual HD content in the series does not justify the £40 price tag but this series does need to be owned so i would say save yourself £23 and get the DVD version, because all BD player will upconvert DVD's and that is about the same quality you will get from the blu-ray version, if you are looking to show off your HDTV i would suggest picking up the pirates of the Caribbean films (easily the best looking blu-rays to date!)
i gave the rating 4 stars because the series is just so good but i would rate the BD transfer 2 stars