Direct debit recruitment via direct mail

Does anyone have any thoughts - anecdotal or statistical - on the merit of including a direct debit form within warm DM packs? I'm interested in whether commited giving ask packs work as well as if it's worth putting a DD form in our general, cash-ask packs.

Previous experience suggests to me that response is negligible, but that could be down to the targetting or the the pack itself.

Many thanks.

Martin Sheehan
Church Army