When I first tried it, I couldn't figure out Preview. I had assumed it would show what the actual post looked like, with smileys and any special effects included, and all I ever saw was the same "writing box" I had used. Then, after a few times, I finally realized that the board takes you back to the little gray box, but that the real preivew is above it!!
Anyway, I thought that Preview would be an exact duplicate of what the actual post would look like. But, last night I used it to make sure that my inclusion of a URL would actually come through as a link. When the actual post was up, the spacing was different. The obvious example was that in the preview, a smiley I had put at the end, didn't fit at the end of the line and was at the end on a line by itself. When the actual post appeared, the text ended in the middle of the line with the smiley right after it.
That turned out to be better, in my opinion, but I didn't like the fact that the preview didn't show it correctly. Some time if the spacing would matter, there would be no way to know how it would actually appear.