To paraphrase a famous saying: You gotta ask yourself...do you feel lucky?
If you decide to make the effort to visit Nemours, you'll feel like you've discovered a French chateau (that nobody else really knows about) that has been dropped into a somewhat suburban neighborhood.
It does take a bit of an effort to get to Nemours, though. If you're traveling individually, they recommend making reservations (with detailed instructions on the website about what sort of message to leave, including how to be reached and at least two dates to visit). If you're traveling in a group, reservations are required and no young children (I want to say either under age 12 or age 15) allowed. The tour will also take some time and I'd say you should budge at least 2 to 3 hours at minimum.
This is bound to deter some people but if you can do it, I cannot recommend it highly enough. I have to admit to being a history geek, though, and if you want history in northern Delaware then it's probably tied up with the duPonts.
But you don't have to be interested in history to enjoy a visit to this mansion: an interest in architecture or gardens or just a vivid imagination that will allow you to imagine being transported to the French countryside should do it.
This mansion is one of the most fabulous, awe-inspiring, jaw-dropping places that I have ever been to, bar none. It rivals for sheer majesty the finest palaces in Europe. This is a place that even the pictures on their website don't really do justice. Yet, strangely enough, it's also a place that you can imagine actual people living in because many of the pieces of furniture were picked because the family liked them not because they looked nice (although of course the are fabulous). The gardens are equally worthwhile. You could imagine yourself a member of the European nobility as you stroll the grounds.
The only bad part is that I read on the website that it is currently undergoing a serious renovation. No telling when it will be done but when it is finished, you deserve the chance to visit.