I like most livebearers, as they seem to pack a lot more personality into a small body than most of the community species. Also, they are versatile. You can keep them in most water conditions, and you can have a big group or just use one or two to set off an aquascape... they usually look good whether you're keeping them for their own sake or for the sake of a particular tank.
What I don't like are the overbred livebearers - you know, the guppies that have L-bends in their spines by the time they complete their growth, and can barely drag themselves through the water due to the excessive weight of their fins. I think it's cruel to breed them that way when their quality of life (compared to wild or small-finned fish) is obviously greatly reduced. I don't enjoy watching a fish tank if it contains animals that are suffering.
Personally I think some of the rarer livebearers (like some of the goodeids) are fairly ugly, but there are also some gems among them. I guess it's just down to individual taste really.