Let me first just say that my stepmom is Thai and has her own Thai restaurant in Bangkok that is doing very well. I like to think I know how Thai food should taste.
Suwanasak reminds me of Thai food that a family in Thailand would have. I went there with my girlfriend and we ate at their sparse, 5 table restaurant. You don't go there for the atmosphere; you go there for the food. It took a while once we ordered (there were a lot of people coming in to pick up their take out orders in the duration), but it's worth the wait. We noshed on a few fresh spring rolls that tided us over which were good. Nothing fancy or huge, and included a couple of sauces. My girlfriend's yellow curry, something she normally doesn't care for, was delicious, and my Pad See Yew was incredibly tasty. The staff was very friendly and most didn't speak English. Typically a good sign.
I'd highly recommend this place if you're looking for authentic Thai food and are not in a hurry. I can't speak for take out orders, but I know you'll probably need to allow for a lot of time for that as well.