I suck at math (quantitative) so I heard some good things about Delta course. I've gone through the study material and it's ok. It did enlighten me about certain kinds of problems I had never known how to do before but as I just started my GMAT preparations a few weeks ago, I'm not sure if I would have seen the same material on the other mainstream sources of prep.
I'm working my way through the questions right now, and I actually find the drilling quite helpful so far. So in my case, it was an "ok" investment.
Why I DON'T recommend Delta course is because they have non-existent customer service. Once they have your money, they don't give a flying hoot what you email them about. They WILL NOT ANSWER. I've sent several because of a password problem and I haven't seen or heard anything but the crickets outside the window
You may want to invest in Delta if you feel you need the extra prep. Just don't expect them to follow through on customer concerns once you've paid. For the record, I will never do business with this company again.
ps. signing up for their GMAT math question of the day isn't a bad idea though.
So a double digit score is a bad thing?