I guess I'm kinda late to this discussion, but whatever.
The first time I went to Korea was in 2010 (back then for vacation), afterwards I even lived there due to University. So in total I've spent about 18 months in Seoul?
If you plan on going to South Korea solely based on your love for K-Pop, I wouldn't recommend going there. Yes, there's posters, advertisments and K-Pop music everywhere (and I even got to see most famous idiols back then, since they perform at University festivals), but I know many students who stopped being crazy about K-Pop after having lived in Korea.
With regards to the service in stores and restaurants, I tend to disagree. Yes, the part-timers in convencience stores tend to be unintrerested in performing a good job, but I never felt offended by it. In restaurants, I never had a problem either? On the contrary, I always was greeted very friendly and often received some form of service. Even when I wasn't greeted at first (due to business or w/e), I simply smiled and greeted them instead. It really helped since I often got some sort of service and very attentive servers.
Korea is still a largely homogenous country, so unless you make the first move to show them you aren't much different from them, many people will simply be hesitant. So it's not that the service in stores, restaurants, hotels, taxis etc. is horrible, it's simply a cultural barrier. Korea isn't a perfect country. There are many rude, even racist people. I was inappropriately approached by bunch of men... but honestly, is there ANY country in the world that has an absence of such people? I highly doubt it.
Also, a factor that highly contributes to any stay in Korea is the language. The first 3 times I went to Korea, I didn't understand a single word of Korean. It was only after I learned a significant level of Korean (was TOPIK 5 when I left) and Korean culture that I realized the huge difference in the way people will treat you.
Finally, yes Seoul is smelly and the smog was horrible during winter. :)
