I ate here over four years ago and I remember the food being decent, but nothing spectacular. I of course had a masala dosa (how can you go to a place called Dosa King and get aloo gobi?) which was a nice treat, given that South Indian food is a rarity in Bangkok, and Thailand in general (North Indian food, on the other hand, is pretty easily found in the big cities of Thailand).
While the dosa was decent, I recall it being not quite worth the trek from Khao San Road (the young, hip, cheap touristy area). Dosa King is in the steel-and-glass downtown area of Bangkok and will require a long taxi ride, unless you're downtown, in which case it's easily accessible by the Sky Train and subway.
The other issue was price. I don't remember the exact price of the dosa, but I think it was in the Bt. 100-200 range (US$3-6), which is extremely expensive by Bangkok standards. You can get a delicious, filling vegan meal at a Thai restaurant for Bt. 50 (US$1.50), so Dosa King was at least two more expensive than a regular joint. That said, by American standards, it's still cheap, but you have to think proportionally to figure out value for your money.
This place is somewhat upscale, so it's good if you're looking for a fancy change of pace from Thai street food and hole-in-the-walls. Or, if you're looking to try something different than tofu pad thai and green curry, it's a good alternative.