DU worked great for me on my recent GC vacation, and they were able to get me the DC discount. They can only get DC discounts on packages though, to get the discount on room-only reservations, you need to call DL directly.
As you know, WDW packages generally aren't "worth it." However, I've found that DL packages at least break even, but often come out a little ahead of the individual package components. And the packages include things that are very usable, park hoppers, a WDTC lanyard and pin per person, $10 per person ESPN game card, one park breakfast (which for a small fee can be upgraded to a resort character breakfast).
As far as discounts go, GC discounts are hard to come by, but DL and PP discounted rooms are sometimes found on Expedia and Travelocity, so it might be worth checking there first. I think it depends on the dates though, really hit or miss.
I'd also like to say that I highly recommend GC!!! I've stayed at DL Hotel, and though it was enjoyable, GC almost gave me the same feeling as staying at a WDW resort, and there are just so many great details and subtle Disney touches!
