People who have historically been on this forum have neglected a lot of well rated shows. The good bulk of the members just came here for Naruto / Bleach and a few of the shows in a particular season at best. And since this isn't an action series, then chances are that this type of show wouldn't have been a priority based on ratings and recommendations alone.
So yeah, I think the best guess should almost always be on the side of "not already seen this".
There are so few of us now though in General Anime, so maybe on that context yeah there's a good chance that many of us have indeed watched it... now that we tallied 2 more (me and David). But even then, I still remain doubtful. When someone asks for suggestions, Kaleido Star rarely comes up from my experience. I think it's generally overlooked.