some people claim, that there are no good restaurants in this area...just overprized low quality

that's wrong. there are a couple of the town's best restaurants to find between Asoke and Ploenchit al lower Sukhumvit. I just would like to mention some Thai restaurants:
Na Aroon Soi 1
Prai Raya Soi 8 (for me the best southern Thai food restaurant in town)
Madame Saranaie Soi 19
in Soi 23 even a cluster of Thai Restaurants: The Local, Oranuch and Wanakarn

for simple but good Thai: Suda Soi 14
or Cabbage & Condom, an institution in Soi 12