I pre-ordered from Amazon back in January and bought a copy on iTunes last week. Interstellar is released tomorrow, March 31st, 2015. I highly recommend you grab the Blu-Ray version of this film. After I watched it twice in 70MM IMAX, once in 45MM IMAX and once in standard Digital theaters and now in 1080P on iTunes last week, I can say that the iTunes version was the least powerful of the versions I saw maybe slightly edging out the digital theater quality that I saw on week one but just barely.
If you have a Blu-Ray player, grab the physical copy of this movie. Uncompressed video is the way to go with a film of this scale and detail.