The Tholians have long been one of the most interesting of the original series alien species, one that over the years has been elaborated on and added to in both printed form and in the various TV series that followed the original.
In that time they have grown and developed into a species with as rich a background as any of the more familiar alien races such as the Vulcans, Klingons, Romulans, and Cardassians.
There is something about them that has always made them both interesting and menacing at the same time. A species that due to their very alien nature will always fail to understand us. One that can always be counted on to make a difficult situation even more difficult.
Maybe that is why we love them so much, or maybe it is simply that they embody the most basic belief that aliens are very, very different from us. Regardless, they will continue to be a source of danger and mystery and one of the few constants in an every changing universe.