The HD DVD add-on for the Xbox 360 may not be main stream by any means, but it's still showing some respectable numbers for an add-on. The NPD Group has released some attach rate numbers that may surprise.
Since being introduced, the HD DVD add-on has sold over 269,000 units. That means that 3.4% of Xbox 360 owners have the device attached to their console. While this isn't a large number at a glance, it shows that the device has been embraced by enough users to justify continued production.