Bottom line - I have been eating here for over 14 years. My wife, myself, and my two daughters LOVE their food. We have only eaten in the restaurant once or twice. We almost always opt for take out. The decor in the restaurant is simple, and not really very impressive. People come here for the food, not for the atmosphere.
The menu is extensive. We have tried many of the various dishes. Anything that sounded good on paper was equally good in reality. We have never been disappointed. Yes, you might have to wait for them to prepare your meal. Quality takes time, and there are a lot of loyal customers who are calling in their orders. So, even if they don't seem busy with patrons in the restaurant, they are super busy back in the kitchen.
My family and I have tried almost all of the Thai restaurants around town. We have found a few good places, but we always compare it to Thai Lahna as the gold standard. I have a dream of having a big party, ordering everything on the menu, and trying every dish that they make. I guarantee that it would make me the most popular guy on the block, and I would get a chance to try some of the dishes that sounded good, but I never had a chance to try.
To sum it up, the decor and the atmosphere are average. The wait time can be a little long. But, if all you care about is the food, then you will love Thai Lahna.