Well, Series 7\'s are supposedly made from the best kolinsky sable. A good brush will hold an incredibly fine point, will not fishtail and will not get a hook like a synthetic brush. They\'re soft, giving excellent control, but will return to that fine point after each stroke. They can hold a lot of paint, feeding it smoothly and consistantly to the point.
Of course, this should be true for any top quality sable brush, and some say that the Da Vinci Maestro brushes are better than W&N Series 7\'s. Series 7 definitely has the reputation for being the best.
But all this is just technical stuff. Get one, and get used to it. I enjoy using them - painting with a Series 7 is definitely a far more enjoyable experience than painting with anything else I\'ve tried. Sometimes, I could almost paint solely for the pleasure of using such a fantastic brush.