We got discounts on our Ultimate Park Hoppers (aka unlimited magic passes aka length of stay passes) without problems. We did not use it for anything else. The American Express White Glove Treatment discount is better than the DC discount at the World of Disney at Downtown Disney, so we used that. We tried to use the DC discount at the House of Blues, but were told we could not because it was only good until 4pm. This restriction was not listed anywhere in the documentation that I could find. I had even printed out the website pages with the offers I might be using to make sure that I knew the restrictions. As it turns out, most offers have a vague, yet powerful, statement that says that there may be additional restrictions and these restrictions don't appear to be detailed anywhere. If you do plan on using the card somewhere, I would plan on confirming ahead of time that the discount will be valid or if someone could provide information on what any additional restrictions may be.
For a discount buying the Disney Club card, I believe entertainment.com has a discount for current Entertainment book holders. I'm not sure if it's still up, but it was valid earlier this year.