I've only used top-whorl spindles. I like them because I can whip it off my leg to get it spinning fast when I'm cruisin' or doing fine yarn that needs lots of twist. Seems like a bottom whorl wouldn't like the shaft being rolled against the leg, since the yarn is running along the shaft; but I've not used them, maybe one could still do this.
BTW, CD spindles are great, good balance and weight. The wide diameter gives them good momentum, and yet they're not so heavy that they drop like an anchor. I dont' know where one gets the rubber insterts recommended to hold them on a dowel, but I've used masking tape around and around the dowel until it was thick enough to squeeze the CD on it. Not perfect, but it worked!
KnittingHelp Queen Bee
It is not because things are so difficult that we do not dare; it is because we do not dare that they are difficult.