Petzl
Freino v1a

Most designs of bobbin descenders require a separate brake carabiner to provide additional friction and control. Besides the hassle of a second carabiner, if the bobbin becomes rotated it can catch under the brake carabiner - eliminating the extra braking friction from the carabiner (and part of the bobbin's bollard) and placing the climber into an uncontrolled descent.

A better solution to using a standard carabiner as the braking carabiner is to use the purpose-built Raumer Handy. The narrow shape avoids catching the bobbin under heavy braking, and has the additional benefit of a narrowing V-shape to increase rope friction while braking.

Instead of requiring a secondary carabiner, the Freino provides an attached brake spur, suitable for use with bobbins, or lever-box belay/rappel devices (like Petzl's I'D or Grigri). This will cause the rope to run out of plane with the bobbin's rope path, introducing twist during descent. In my experience, this amount of twist is negligible when rappelling from typical exit points in tall trees, but will increase wear on the side-plates of the bobbin. It would certainly become more of an issue in stiff ropes, longer free hanging rappels, and drops rigged with excessive rope.

I have used this device extensively in tall-tree rappels in conjunction with a Petzl Stop and find it well suited to climbs where I am not as concerned about weight. Because this carabiner is specifically designed for use around running ropes, I would personally prefer if the locking mechanism was triple action or screw lock to avoid accidental openings.

Petzl has released the Freino Z, a newer version with additional locking options. The spur is oriented differently in these versions which requires rotating the spur through the attachment hole of the bobbin, which is not compatible with older versions of Petzl's bobbins (the Stop and Simple). Unfortunately, I have an older version of the Petzl Stop myself.