I've used apt, and I like it. I think that the reason that I use yum now is that it's what ships with Fedora, and I'm trying to support it so that it will improve. I know that skvidal is working on making it a lot better. In particular, he's working on making the metadata (the headers that you have to download) work a lot better. It probably won't be in Fedora Core 2, but it might make it into FC3.