Let me try to clarify my thoughts a bit.
I don't get to see many previews. When I do, I get to see them on Tues. or Wed. of the week when they are set for broad release.
In situations like that, if I think a movie is worthy of a review, I'll write up the review but not post it until Friday -- the day when folks can actually go see it. That still gives you enough time to make up your mind as to whether you want to go see the movie.
As it is, I only go to a handful of previews a year, so it's not as though I even have the option to post early reviews all that frequently.
In the situation in this thread, Felli is posting a review written by someone else for a film that is currently showing in only 2 cities, and 4 theaters, in the USA. I think it's bad form to post a review for such a film, when most on this board have no chance to see the film at this time.
It did not help that in his post Felli put down The Force Awakens discussion, and also stated that "The Revenant is the MUST see movie of the year" -- something I seriously doubt he can claim since he's yet to actually see The Revenant.