I LOVE Nielsen's. Their custard is exquisite (if pricey at $4-5 a 16-oz concrete), the place has a wonderful old-timey 50's diner atmosphere with black white and red decor, and they do a great job keeping the place spotlessly clean (including bathrooms). For newcomers, the menu can be a little confusing (I had no idea when i first came what custard even was, as opposed to ice cream). Rather than outline it here, my suggestion would be to go there and sit yourself down with a menu, where on the back you will find plentiful praise for their delectable product. If you still have questions, just ask the scoopers. They know what they're talking about.
Pros: the custard!
Cons: loud machine at times, and custard prices add up faster than you think.