The Petzl Pirana is a one of the first descenders specifically designed for canyoning. It has multiple friction points (2 above & 1 below) and can be used with any rope diameters from 8 - 13mm. It gives a very smooth descent and does minimises kinks on the rope. Another convenient feature is that any locking karabiner with a 12mm cross-section diameter can be slotted through the unit making a pretty fixed descending unit (no more worries about losing a standard fig 8/bug); this is especially useful when descending into fast moving water.
The Pirana has a number of braking and locking positions depending on whether you are single/double roping. By adding an additional loop on the rope and making use of the braking spurs, the friction can be increased/decreased based on pitch size/rope diameter/general conditions. Make sure the karabiner is used in the descent system.
As regards locking, the upper braking spur can be used to temporary/longer term lock the gear. As can be seen in the figure below, different locks can be used depending on whether you are free hanging or in contact with grounding. Nothing worse than a un-releasable locked descender ;-)
Overall, the Petzl Pirana is definitely a must for active canyoners. It conserves the canyoner's energy on pitches and can also protect the rope a lot more than other descenders. Plus, its ease of locking/braking modes make it great to use in any canyoning situation.
Pirana images & technical information © Petzl 2010 (http://www.petzl.com/en/outdoor/descenders/pirana)