Very rarely nowadays, mostly on long journeys when I'm hungry as they have good coverage across the country. Can't really go wrong with those cheap double cheeseburgers, good value and I might partake in McFlurry as well if they have a flavour I like. Wraps are ok too, normally just nab the special one for the day for £1.99.
I was actually having a conversation with someone about them the other day, I never really saw their burgers as burgers per se, more burger like sandwiches. Patties are just too thin for me to think of anything they serve as a proper burger, not to mention size in general. They still do the job and consistency is one thing McDonald's do well, can't say I ever felt any snobbery towards them but not really a place I go if I feel like treating myself with a cheat.
I do think their saving grace is coverage and now opening hours, three 24 hour branches within 15 minute drive away from my house and seeing how a lot of food places shut around 10pm here, just about the only place to get something after that time. Their breakfast is pretty decent though but I don't eat breakfast in general so very rarely eat that.
Speaking of consistency, it's probably why I can count on my hands how many times I've eaten at Burger King, only one branch in my hometown and it's always been a bit on a shabby side and messy looking. That and I literally saw them microwave a whole burger in a wrapper in front of me one of the first times I've ordered there, fries were not fresh and think that experience just put me off them in general, especially when I could walk few minutes to McDonald's.
I did eat at BK bit more when I lived in US but the menu was better (as was McD's) and helped I walked past it to and from work.