Cliff, it is indeed in the beauty of the hamon that its true value rests, and the makers who recognize this seem to do the best with it, but the hype around differential hardening has provided some of the biggest honesty challenges for many a maker in representing their product and methods. Doug, very good explanation and I am glad we are pretty much on the same page on the general conclusions about hamons, there are a few additions I would make since I have spent some time studying these things and have found some tidbits beyond the conventional wisdom of the topic.
No other phases in steel can match martensite for possessing both strength and toughness at the same time, pearlite can lend toughness but only via ductility and at the expense of strength. A soft back blade cannot be stronger than a fully hardened one, but it can be more ductile and thus tougher, but less elastic. In other words a blade with a pronounced hamon will not flex nearly as far without taking a permanent set but will then permanently bend farther before breaking, but the bend will happen with as much as 1/5 the force it takes to bend or break a fully hardened blade. Those fantastic old katanas that everybody defers to when tauting the superiority of the hamon are notoriously easy to bend, they just don't include that tidbit in all the hype.
Also the guys who are emotionally invested in hamons will like to tell you about how "stiff" the blade is and how much force it takes to "flex" it but that is when you need to ask them about how thick the blade is. The amount of force required to elastically flex a blade and still have it return to true is determined exclusively by the thickness and shape of its cross section, heat treatment only comes into play after you have pushed the blade too far in flexing it, and pearlite will be pushed too far long before hardened steel. So a blade that is differentially tempered will actually possess a higher toughness along with a higher strength than one that has a hamon from differential hardening.
Of course the very edge of a differentially hardened blade will not be any better than a blade hardened any other way, and in fact the chances are greater that it could be not as good if the maker gets too caught up in the appearance of the hamon instead of keeping the properties of the edge their #1 priority. Here are just a few examples of how the hamon can be in conflict with a good cutting edge:
If the maker feels only water will give the most striking hamons with modern steels that cannot handle that quench, they will end up under-soaking to accentuate activity, and avoid cracking.
If the maker doesn't use clay but merely edge quenches into oil with simple steels, thus leading to many under-cooling issues.
If the maker applies clay too close to the edge and causes pearlite colonies in the critical edge martensite. This will not necessarily be visible until it reaches much greater volumes ( I have spent many hours studying the inside of the habuchi area under the microscope on both ancient and modern swords)
One that I find most disturbing is when makers are using methods to nail exact soaks at exact temperatures for their given alloy, but abandon them for methods that produce a better hamon regardless of the advantages they are giving up for edge quality.
Along with these, and others, it is a bad omen when makers start pointing to a hamon as proof of a superior heat treat when the only thing it says is that the heat treat was good for hamon. Good hamon and the optimum heat treat for that steel are not necessarily the same, especially when you consider that in order to form a hamon you must do many things that are contrary to a good heat treat, e.g. intentionally making pearlite is not a rational thing to do in any other hardening operation unless you are making a hamon.
Just as a good hamon is no guarantee of a good heat treatment neither is its presence an indication of a poor heat treatment, there are many makers who have their priorities all in line and make stunning hamons with excellent performing edges. Burt Foster, Rick Barrett, and Dan Maragni are just a few of the makers who I would fully trust their edges on blades sporting hamons that are outstanding. These guys know their temperatures, know their quench oils and have worked out exactly how to apply their clay for those mediums. But above all you will never hear one of these guys feeding you a line of bull about how super their blades are just because of the hamon, and since they are able to recognize the limitations they are able to easily work around the pitfalls to produce an excellent edge.
Edited to add: This is one of those topics that is very touchy and can easily brig about hard feelings, the personal investment that so many have in developing their hamon will cause them to take any contrary physical facts rather defensively, and that is too bad, for such a beautiful addition to a blade should have the benefit of rational, objective and factual information to rest on, and be honestly appreciated for what it is.