I watch may SD channels on my DLP TV, and I must say the SD picture quality is the best I've experienced (I've had Comcast and Dish Network). Granted is no where near HD quality, but it is definitely acceptable. The cost difference is $4/month more for the HD box.
Right now the biggest downside of SD on a large DLP is the screen ratio. The SD box won't provide 16:9 aspect ratio, so unless you use zoom on the TV, which worsens the picture, the will always be black bars on the sides. Also, in the future there will be many more HD channels, which means more to miss out on with just a SD box.