I've spent my life jerry rigging solutions for rigging, logging, arborist work, wheeling, and all sorts of other redneck activities. I've never been able to figure out why a come a long isn't a critical part of every wheeler's recovery gear. I wouldn't leave home without one.
The models I'd put at the top of the list are the More Power Puller mentioned above which is an American classic, and the Lug All units:
http://www.lug-all.com/pages/cableratchetov.php
You want a solid cast unit, while keeping the stamped steel to a minimum. I've always been tempted to switch the standard cable to synthetic, it would seem you could probably add more line length by doing so as well, especially with something like a MasterPull line which is basically compressed.
I've used a come a long for a multitude of reasons when wheeling, particularly when traveling alone where a single front mounted winch can certainly use supplementing. Recovering one's vehicle backward out of mud or off a rock or log when moving forward has been deemed unadvisable, or pulling the back of your rig sideways away from or off of an obstacle are two good application examples. And if your rig won't run, a come a long is going to move it to solid or flat ground more effectively than a winch powered by a rapidly dying battery. They're particularly useful in expedition wheeling, where solo travel is common and self sufficiency is paramount.
As someone mentioned earlier in this thread, I think a comprehensive article discussing the wide range of come a long applications and reviews of some of the best ones in an upcoming issue of OJ would be a great idea. Maybe I could consult!