I use the best products from different ranges, my kit contains mostly Benefit, Chanel, Stila, MAC, Pout and NARS.
Sticking to one brand wouldn't work for me and I don't know one professional makeup artist who it would work for.
IMO, and many other makeup artists' opinions, both here and in the US, the quality of some of MAC's products have declined since the takeover by Estee Lauder.
Have a good read of this thread -
http://www.salongeek.com/skin-geek/54896-mac-makeup.html
Kevin James Bennett, a NYC MUA with bloody loads of Emmys!, has created a web site which gives info/reviews/advice on serious professional products. There are no MAC products on this site btw....
Makeup Artist Education and Workshops - Makeup School - Professional Makeup and Cosmetic Product Reviews - Kevin James Bennett - Emmy Award Winning New York City Makeup Artist - Film, Video, Photography