I did this sketch of David Austin roses to test out some new brushes.  My old brushes were not working for me at all.  I was pleasantly surprised how much I could do with the new brushes.  I did this sketch in about two hours.

My students and I have been using Isabey mongoose brushes for quite a while now, and they have been great, making a fine point and holding up well over time.  Lately we have all noticed that the newer ones have not lived up to our expectations.  I contacted the rep for Savoir Faire and found out that the 6100 series is now made from mongoose-like bristles.  The reason it seems is that the mongoose is now a threatened species, so the company felt compelled to make changes.  I don't know the whole story with the mongoose, I do know that several manufacturers are still using mongoose in their brushes. 

I have tried several other brushes and found some good ones.  They make a nice point, or chisel in the case of brights, and have a lot of control with a decent amount of spring.
 
Raphael Kevrin The rep from Savoir Faire sent me a complementary Kevrin Brush  It is made in France of mongoose.  I tried a round and a filbert.  This is a very nice brush. You can get Raphael Kevrin from Blick.

Kalish  I tried a round and a bright mongoose.  They are very nice, a little thinner than the old Isabey. They are handmade in Ireland, but the price is about the same as or less than Isabey.  The numbers and sizes are consistent with other brands.
http://www.kalishfinestbrushes.biz/

Kolonok  This is a Russian made red sable.  It is about the same price as mongoose, although not as pricey as kolinsky sable. I bought two flats, size 10 and 4, and a size 4 and 8 round.  These are really good brushes with a lot of control.  In the past I have found that sable does not hold up, but we'll see how these do.  They also carry watercolor brushes(shorter handle) and kolinsky sable oil painting brushes. 
http://www.kolinskyartbrushes.com/