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
"There is a vitality, a life force, an energy, a quickening that is translated through you into action, and because there is only one of you in all of time, this expression is unique. And if you block it, it will never exist through any other medium and it will be lost. The world will not have it. It is not your business to determine how good it is nor how valuable nor how it compares with other expressions. It is your business to keep it yours clearly and directly, to keep the channel open. You do not even have to believe in yourself or your work. You have to keep yourself open and aware to the urges that motivate you. Keep the channel open."
— Martha Graham