1. If not all of your team is truly convinced to DCI, then don't start with that. Even if only one person in a team doesn't get DCI it can make it very hard. As with every "new" thing, it requires more education. Experiments are fine, but at some point the whole team needs to have a common "mental model" of the system you're working on.
3. Extending objects with roles should be at the context level. We've had some extending at the model level (due to the lack of IdentityMap in Rails 3.0, now that we've upgraded to Rails 3.1 this problem disappears). This quickly leads to a mess. It's also a problem of using ActiveRecord "objects" with DCI. Use ActiveRecord only for persistence, not for domain logic.
4. DCI without contexts/use-cases is useless. We were not disciplined enough to have contexts everywhere and problems appeared quickly. I need to stress it more - DCI is not (only) about dynamically extended objects. It's much more than that. You need to start thinking in use cases.