I've been to the Musee de l'Orangerie and it had a wonderful small collection of Impressionist works, but it has been closed for renovations for several years and still isn't open (should be in a year or two). Their collection is quite good.

There isn't anything particularly bad about St-Denis area around cathedral or many other parts of the town with main attractions. The cathedral is worth seeing in itself just as much as almost anyone is, actually. It is an important part of French history, has some of the best French funerary sculpture extant, and the stained glass windows predate Notre Dame and Chartres, I believe -- the abbe Segur's patronage and interest led to the design and quality of stained glass in that church. This church is one of the first Gothic masterpieces, as are the windows.

I also found the tourist office staff very friendly, and it's a very short walk from the metro stop. I wouldn't go without seeing some of the other attractions in town, myself. There is a lovely park, but their museum is very good, and has a large collection related to the French commune which is unique--the museum in the former Carmel convent (museum of art and history of the city of Saint-Denis). Here is a link describing it:
http://www.museums-of-paris.com/musee_fr.php?code=529

There is also a Christofle silver plate museum and workshop, as I understand it (tourist office will have info), but I did not have time to visit that, unfortunately, with all the others things.

christina, thanks, as usual, you provided some great info.