[Dark Matter]

I cannot speak to the tactical use of these devices. However, I do have a lot of experience working with them.

I personally carry the Black Diamond ATC Guide. I think it's simply the best, most versatile device on the market. I haven't worked with the new Reverso yet, although it looks to be top-of-the-line as well. The older style Reverso had softer metal that wore through over time. My old one is very sharp on the load end from repeated raps (but I loved it!).

In addition to several of the points brought up by other members, I would like to stress a couple points. The auto-locking feature of the ATC Guide (or Reverso) is incredibly useful. First, it allows you to build your belay off the anchor instead of off yourself. This is especially useful in a rescue situation, as you no longer have to escape your belay in order to initiate further rescue actions. It also gives you some freedom of movement, which is positive for your comfort, as well as allowing you to be more efficient by hydrating, eating a bar, adding/subtracting layers, checking topos, etc while your second is climbing. Furthermore, it allows you to belay two seconds simultaneously, which is far more efficient than climbing one at a time on two ropes or climbing with a 20' 'donkey tail'. It does take some good rope management skills though...

As for rapping, I almost always rap on an extension unless I am cleaning sport pitches. This moves the device away from my body which allows for several benefits. There is little possibility for sucking loose clothing, straps, or hair (not a problem for me, but some people...) into the device and ruining your day. The extension also doubles as your method to clip into the top anchor. It also allows you to place your autoblock (Roger_Out's dead man's catch) on your belay loop instead of on the leg loop, which considering it's my back-up system in case something else goes south, I want it to be full-strength. This also lets you use both hands for braking. Finally, there is a nifty guide trick to turn your ATC Guide or Reverso into an ascender in a matter of seconds if need be (I have actually used this quite often to free stuck ropes and in crevasse rescue).

I think the Gri-Gri is great for sport climbing. It's low drag to pull when belaying and locks off. However, I would only recommend experienced belayers use it (which is somewhat contrary to popular opinion). This is because when used improperly, the device loses all friction. I have corrected several people who were holding the device in a way that could lead to a ground fall. It is also bigger and bulkier than the ATC Guide and less versatile. For multipitch or alpine I typically carry the ATC.

If there is going to be little vertical terrain I might rely on a munter hitch. I don't like the munter for long stretches of vertical ground because it puts twists in the rope, but it's good to know as a back-up for most anything. Most rescue systems take advantage of munter-mule combinations to create a load-releasable hitch that can be tied off and be safe to leave (if you need to rappel down to someone, for instance).

Back to pure rappelling, if you have little or no reason to belay, the figure-8 is smoother and faster than ATC's or most other rappel devices. I think Freedom of the Hills has a table of different devices and their pros and cons.

About 80% of climbing accidents occur on the descent. A vast majority of accidents in Accidents in North American Mountaineering are lowering and rappelling accidents! Don't get casual!