dstiles - 10:47 pm on Mar 15, 2012 (gmt 0)
Most browsers IE, FF, chrome, Safari etc) begin their UAs with either Mozilla/4 or Mozilla/5 depending on browsaer age and a few other things. It's a stupid historical thing brought about by MS wanting to crash the web scene.
If you block most Mozilla/4 or 5 UAs you will lose a lot of real people.
I really recommend reading though the past year's Search Engine Spiders forum.