TLDR ; not the same level as Pai/Khao San Road in terms of quality, but still very good for the price and portion size.
I've been here a few times now and tried a few dishes. #1 tip - order the Islamic noodles (their "secret menu" item).
Their pad thai, green curry and thai fried rice are also good options for mains. I was quite disappointed by their khao soi though. I would go to Pai or Sukhothai or Khao San Road if you're looking for good khao soi.
If you are looking for a dessert that's not too sweet, give their mango sticky rice try. Salad King serves a decent sized portion compared to a lot of places.
Overall Salad King has great quality food and big portions for the amount that you pay. There is also lots of seating so even if there's a line, it moves quickly. The only complaint I have is that it's quite noisy so I wouldn't recommend if you want to have a conversation during your meal.