Williamsburg is a seething mass of Thai restaurants that have given rise to their own sort of Brooklyn Thai cuisine culture swapping and sharing chefs, employees, inspirations. . .
Tai Thai is relatively a newcomer to the scene already dominated by Planet Thailand and Sea, but has done well for itself, already expanding its dining area and finally scoring a beer and wine license (sorry no more BYOB!).
I've been to Tai Thai on several ocassions, but would only recommend the $4.95 lunch special, as when paying full price the Thai elsewhere is better. However, for a cheap lunch the fresh veggies and lightning fast service of Tai Thai can't be beat. It includes an entree of curries or stir-fries with brown or white rice, or noodles, and a small veg spring roll. The white rice tends to be very Minute Rice-like, so go brown if you care.
Other Thai in the area includes: Planet Thailand (the original), Sea (an affordable spectacle), Chai (my preference), Cheers (nice outdoor garden and down home feel), Siam Orchid, Amarin, Hao Sam, Lily Thai, OTT, Moon Shadow, Thai Cafe.