I've got a set of cheap brushes that I use for speed painting. I love em - great bang for the buck ($1.50-$3 per brush, Robert Simmons White Sable, from [url]www.dickblicks.com[/url]). I have a few higher quality brushes that I save for special figs.
I end up having to replace my everyday brushes about once a month, so I don't like to invest much in them, especially for speed painting my space marines where I don't care about quality as much (I've got two armies - Daemons for painting, Space Marines for letting the new players/kids borrow, or when I want to paint without worrying about the details so much).