For a "hole in the wall," Sakura Express has a nice placement on the same block as the Princeton public library and across the street from great Princeton establishments like Alchemist & Barrister and The Ferry House. I had no idea this place even existed, or that such a place could exist on Witherspoon Street, until my son had a craving for unagi don and sent me to fetch it for dinner. The outside is nondescript and because the seating is a bit above street level, you can't really tell what it would be like until you walk in.
The tinkling of a bell, the chatter of diners, the smell of sushi rice, and the warm air wafting from the kitchen greet you as soon as you open the door. Sakura Express does its best to maximize its tiny square footage, cramming a little bit of kitsch here and there, offering only bar stools to sit on so diners will hurry along after eating, a simple salad bar where you can help yourself to any size dollop of sesame or ginger dressing you desire, and the main stage where sushi is made and orders are taken. From the postcards lining the wall underneath the larger than life menu and from the familiarities and niceties exchanged between the ladies next to me and the owner, you can tell that there are many loyal fans of this tiny place.
The food is priced alright for what you are getting. My daughter and I shared a shrimp & vegetable tempura for $9 and my son's unagi don was seasonally priced at $14. There were generous portions in both dishes, so you certainly don't feel jipped, and while the tempura was just okay, the unagi was just outstanding. The eel is served with skin intact, so it's definitely not for first timers, but the avocado slices certainly help.