Honestly, I've never tried MAC brushes and don't care to. However, I do use MAC dupe brushes that are recommended by many people including die hard, obsessed MAC addicts. :-) I got most of my brushes from online as well as Michael's craft store.
Here are the links to the MAC brush dupes as well as a youtube video that tells you which brushes to get that are considered MAC dupes. You will save so much moolah! Seriously, for the cost of one of the MAC brushes, I was able to purchase ALL of the MAC dupes and some of them in pairs. Someone (can't remember where I saw this) did a recent calculation of how much money you save and it was over $500. Um...yeah. I have been using these brushes for several months and I LURVE them!
These are the brushes I use daily:
Loew-Cornell 1/4" mini mop = MAC 217
Loew-Cornell 1/4 Maxine's mop = MAC 228
Loew-Cornell 3/8 Maxine's mop = MAC 239
Loew-Cornell 1/4" angled brush = MAC 266
I love this guy and he tells you which brushes are dupes: http://www.youtube.com/watch?v=D9lPfKzQAXE
This is an online store where you can order them: http://www.hofcraft.com/loew-cornell-brushes.html
Here's another video that shows dupes with MAC brush numbers on the handles. I think that's a good idea. I might number mine too.
If you have any questions or suggestions, lemme know!