I've got three pipes with horn stems. This is what I have experienced.
Pros: Horn is softer than any stem material I've experienced. It has a great feel between the teeth.
Cons: Horn swells and contracts with moisture and can become slightly rough as the grain swells. Horn stems are cut much thicker because they are so soft. It is also much easier to chew through.
I wouldn't bat an eye if I found a pipe I wanted with a horn stem, but I would definitely treat it a little more gently than my other pipes.