DC restaurant week is pretty disappointing. A lot of nice options don't participate, the menus are too boring, and its not much of a deal.
That being said, the "rip off" places tend to be either places that are moderately priced to begin with or serve small plates.
If you stick to only higher end restaurants that serve more traditional three course meals, I think you can make out ok. Its not exactly half off, but it usually at least works out to a "your dessert is on us" kind of deal.
Looking at the list this year, here's where I think you might be able to get a decent value on a good meal:
2941
Acadiana
Al Dente
Bistro Bis
BlackSalt
Brasserie Beck
Equinox
Kaz Sushi
Mintwood Place
Rasika West End
The Source
Vidalia
Zentan