Note this is a review of the quality of the Blu Ray rather than the episode itself, which you can read lots of reviews of elsewhere and find lots of different opinions.
The Blu Ray quality is very disappointing. Due to some technical jiggery pokery required to allow the 2D and 3D versions to be on the same disc, the frame rate has been slowed down from that which was broadcast, and this is noticeable, particularly if you saw the broadcast version. While the picture quality itself is good, at times the motion doesn't seem smooth, and where it is most noticeable is with the audio quality, which is not good.
Before anyone mentions firmware upgrades, I have tried this on both a PS3 with latest updates and an LG Blu Ray player with latest firmware. Reading reviews elsewhere others have noticed the same thing, and the biggest giveaway is that on the box the running time is listed as 76 minutes, whereas when you watch the Blu Ray it is around 79-80 minutes.
My advice would be to avoid the Blu Ray and hope the BBC fix it in the future (and offer replacements to those of us who have already purchased it). I doubt that this will happen though. Probably better off with the DVD and a good upscaling player.....