I ate at Zen Palate a few times during my year of strict vegetarianism and my experience is mixed. There are a few items which were always outstanding but often when I tried one of the specials, they flopped. I've found that the dishes with sauce tend to be soggy and often unpleasant. The noodle dishes are often excellent and I can't recomend the key lime pie enough.
The great location at the east side of Union Square does make it an idea stopping off point for an early dinner, but I would recomend getting there early as it can get rather crowded. If you're ravenous while in the area and prefer strict vegetarian dining, its worth stopping in. Otherwise I'd suggest making the trek out to Angelica's Kitchen, Kate's, or Red Bamboo.