I go to Lemon Grass about once to twice a month. You can't really go wrong with any of their dishes. Lunch prices are cheaper and you get a spring roll, but their dinner dishes are bigger and you're more likely to take food home with you (always a plus to have a second meal). Service is usually always prompt and courteous.
If you're vegan make sure you ask if your dish has fish sauce in it (most don't, but best to check and see). Also, if you like your food really spicy ask for it "Dennis style." They'll add more red and jalepeno peppers.
All in all, I highly suggest checking out this place. But, I wouldn't go too far out of your way for it, since there are comporable Thai food restaurants all over the valley.