I am so glad I found this place!
The 76th Mott Street version (the Cafe) is a small, hidden nook down a flight of stairs. With the small tables, sitting across from a stranger for your meal, it has an sleek, modern cafeteria feel. And its a treasure trove of exotic snacks. They have sago drinks, blood congee, meals with duck tongue, snow frog jelly, you name it. It's a little more expensive than usual for me (my snacks are usually $1.50 or less), but more than reasonable for the average person. At an average cost of $4.00 per item, it's very much worth it. The Restaurant is 20 Mott Street - it's bigger, with a bigger selection of stuffs such as ice cream, and more entrees. I kind of like the Cafe better - it has a more inviting vibe. But they're both worth a visit. 3.75 stars!
2/21/06 Update - It appears that Sweet n' Tart is gone, replaced by a new bubble tea type cafe. (It took me a while to realize the big new sweet and red sign was pasted over the place that Sweet n' Tart used to be. The restaurant down the street is still there...but the cafe? Kaput. What a shame. But - eventually I'll try the new place, and let all and sundry know the verdict.