I do frequent McD's more in Korea because I find the food fresher and tastier compared to the states.
But I do stick to the ones I know - like this one in Youido inside Seoul Finance Center Mall (Youido station, exit3). It's fairly new and has fast service.
The McD's here usually have pretty decent food quality. I get an err sometimes, but not often.
McCafe's coffee is not bad for the price too! Coffee prices here are over $3 for a decent cup in big coffee shops. McD's is a nice cheat for me - I can get a cup of coffee while eating my sundae :)