Well, people HAVE saw it, or at least, a version of it during test screenings a few days after the release date was pushed back last august.
If you listen to mugglecast, one of their members, Erik, got to see it at the Chicago test screening. He gave us some pretty good insight as to what was changed.
If you listen to pottercast, a fan wrote in with a VERY detailed movie review, also having seen it at the Chicago test screening.
We learned these things:
Bill and Fleur aren't in the movie
Luna finds Harry on the train via her spectrespecs. A Quibller product beats out a deathly hallow
no funeral, nor battle. Only the AK curse and the death eaters running away
No "Other Minister" scene
Harry and Ginny don't kiss becuase of the quidditch house cup, she meets him in the room of requirement and they kiss.
Ginny hides the potion book and has harry close his eyes so snape can't use legilimens against him. (then the aforementioned kiss takes place)
The burrow attack interrupts Harry and Ginny when they were about to kiss >.>
No Dursleys/Privet Drive
No inheriting Grimmauld Place and Kreacher
No Dobby and Kreacher spying on Draco
No explanation for Tonks' brown hair, or patronus
No Remus explaining the Greyback backstory
Harry isn't frozen as he watches you-know-who's *winkwink* death from the cracks between the wodden floor above him
movie opened with a scene from the 5th movie, the ministry, to be exact
you then see the office shots from the trailer, and the millenium bridge collapsing
it then cuts to harry at a coffee shop. he flirts with a girl, asks what time he gets to work, he sees dumbledore across the train track we then see the side-along apparition
the scrimgeour "i must not tell lies" scene was cut
no crabbe and goyle as girls
no moaning "MURDER IN THE BATHROOM" myrtle
from leaky: There was definitely tension between Ron and Hermione. In the very beginning at the Burrow, Ron cleans off a bit of toothpaste from Hermione’s cheek. Later in Slughorn’s class she describes what she smells in the love potion and trails off with “spearmint toothpaste…”
that's all i have time for now
Test Screening of Harry Potter and the Half-Blood Prince! Major Spoiler Alert! - Harry Potter discussion forum for movies, books, and more! - The Leaky Lounge
Detailed Report From "Half-Blood Prince" Test Screening - The Leaky Cauldron