You can't go wrong ordering what they're known for, which are their pupusas.
My family and I always order for pick-up though because their service isn't always fast inside. They usually give you time estimates on how fast your food could be picked up, which is nice, but they don't give the full price of what you've just ordered, so keep track of that personally.
Don't get me wrong, everyone is always nice whenever we talk to them, but they're mostly concentrated on getting orders together, etc.