I prefer none at all - just give me a flip chart - but sometimes you can't escape using slides. Readers of this blog know my longing for the days of overhead projectors because those marvelous machines permitted a more spontaneous, less programmed, exchange with the audience. Unfortunately, if you ask for an overhead projector nowadays the clients think your next request will be for a cup of Sanka.
Anyway, I often like to use pictures instead of words. A relevant photo can be a nice backdrop as related points are discussed. Here's one from my class on council presentations:
I note that the picture would convey an effective and subtle message if one were discussing the importance of maintaining a speedy response time for emergency services. The audience immediately identifies with the person on the ground. That photo beats any flashy slide with bullet-points.
And that is one of the aspects of visual aids. You want them to convey and, if at all possible, connect.