About 50%, I'd guess. I have been on a Stila binge lately, and I have quite a few random brand blushes/lipglosses (UD, Bare Escentuals, Revlon, etc). All my favorite shadows are either MAC or Stila though. I'm not really into having all one brand of things, I like variety. =]
I'd say 80% of mine is MAC. Every lipstick I own is now MAC because I packed up all my mostly unused Clinique and Estee Lauder lipsticks, and sent them to my mom and sis. I'm debating sending all my unused shadows to them as well, since I have enough MAC to keep my eyes pretty for a looooong time.
The only non-MAC things I use regularly are my NARS and Benefit blushes, and some Sonia Kashuk brushes I can't part with.
Probably 80% Mac... I have a couple of Dior foundations (Airflash and DiorSkin Forever), couple of Benefit items (Dandelion, Browzing), Blinc mascara and liner, and a couple of other non-Mac items I guess.
I think I'd have to say that about 50% of my collection is MAC (excluding nail polishes, which is like 1%). But I also started collection in August. The majority of my eyeshadows are MAC (75%+) as well as a lot of my blushes/highlighters (MSF=80%+) The rest is a hodgepodge of drugstore and high end. There are so many great products out there, I just don't think I could limit myself to MAC only!
Around 2%
I like MAC, but I like other brands too, and being a jobless secondary school student, I have to shop around for bargains.
If I had more money I'd probably buy more MAC, but I really don't think it would ever make up more than 40% of my makeup collection. There's so many different brands out there, with so many interesting products that I really don't know how anyone can just stick to one brand.
I would say that a good 30% is MAC, 30% is Lancome, 20% is Urban Decay, and then the rest is assorted stuff (Sephora, MUFE, Vincent Longo, Clinique, Estee Lauder, Chanel...).
I'd say about 85% is MAC. It makes up the majority of my stash. The rest is mostly high-end (NARS, Stila, UD, Shu, Cargo) with a few random drugstore bits thrown in. I mostly only use the MAC stuff though.