I don't like them. Bouncing transitions between slides. Flying bullet points zapping in like a space ship. It annoys the audience and does not help get your message across. More over, they are impossible to edit.
There are exceptions:
- Complicated technical diagrams that require a build-up to explain (still, putting them in as a sequence of slides will make your life easier
- Perpetual motions such as slowly turning wheels or a high-way of moving arrows to support a concept of a never-ending force