A few weeks ago I wrote about compressing pastors and others who speak from the platform. I really like to use compression (judiciously), and would use it on just about every input on the board if I had enough comps (one of the many reasons I want a digital board...). Compression can really help even out the spoken word, especially in smaller venues, and enhance clarity.
On the other hand, when over-used, compression makes things muddy and takes away all the dynamic and punch. I came across the video below on Dave's Going to 11 blog. It does such a good job visually and audibly demonstrating the dangers of over compressing something I am posting it here. Check it out—it's really interesting (make sure your speakers are turned up to a good listening level).