I have had my Brompton for 6 years now - my commute is 7 miles each way and I never miss a day - plus, a few long rides during the year (randonneuring). The is in Toronto so similar snow/ice/salt issues as you. In winter, I run Schwalbe Winters (studded), and I run Kojak's for the rest of the year. I almost never clean the bike, but I lubricate regularily. I have several spots with surface rust that I recently touched up (with Brompton's touch up paint) primarily on the rear triangle. I have worn through 7 rims - I use Kool stops or Swiss stops pads. I have had the hinge pin replaced once and I probably have another year left on the one I am riding right now. I have rebuilt the sturmey rear hub twice.
Reliability wise, I don't think Bromptons rank lower than most other bikes - I used to commute with a mountain bike and had as many repairs as the Brompton. I feel commuting year round on a bike is very hard on any bike. All the "cheap plastic" parts that everyone complains about have yet to fail on my bike.
BTW, my latest rims are CSS rims which are supposed to last longer but we will see. As well, if I did it again, I may go for a titanium rear triangle to reduce the amount of rust.