Thumbs down is not positive.
I think the problem is that Ebert is reviewing this movie expecting it to be more harder sci-fi, especially compared to Star Wars. However, he did give it a recommendation and I love the guy so much that I can't even hate him when I disagree with him. IMO, he is a true gentleman when it comes to taste and is well spoken in his blog.
So which is it? Does his 2.5 stars constitute Thumbs down or up? Wait, why do I care? He gave 3.5 stars to the Phantom Menace, for god's sake! This movie has 94% on Rotten Tomatoes and there are people commenting here as if Ebert's review (one of only four negative reviews) alone validates all their prejudice toward this movie.