I came upon Esaan Nation Kitchen whilst wandering around one night in Sukhumvit.
Let me say this place was a good discovery. If you like spicy foods, it's a good choice. I ordered the Tom Yum soup, and it was definitely the most complex and strong-tasting Tom Yum soup I've tasted. The quantity of shrimp was generous, though I was a bit frustrated that many of the ingredients were not edible and apparently were just present for seasoning / spice purposes.
I had some problems communicating with the people here. The food came with some kind of mystery sprout vegetables as appetizers - see the photo in this review. I tried to ask the staff if you eat the whole vegetable, or just the seeds inside. The message didn't get across, but somehow they brought me another plate of the vegetables. A strange experience of good service indeed, but it was not what I was expecting!
Regardless, the pricing here is fair and I think Essan Nation Kitchen merits a repeat visit.