We have used Boston Coach in other cities, but not DC. When we've used them, they have been very good. I don't know how their prices compare. By the time we get to the point of using a car and driver, cost has become a secondary factor.
Here's their website if you want to check them out. I have been told the website is nice to use. Unfortunately, my usage is usually for extremely convoluted situations, so I've had to call them. They've always been patient, friendly and nice to me, so I can recommend them there.
http://www.bostoncoach.com/home.jsp
