Certainly, in the United States, US philately is the most popular. Probably 80% of American stamp collectors collect US and more than half the stamp dealers in this country are US dealers exclusively.
What Can Be Done: United States stamps offer a specialist tremendous opportunity. There are over 4,000 different mainstream stamps as listed by the Scott catalog. There are many rarities, but all but a few of the thousands of stamps issued over the last 65 years can be bought at little over their postage value. Specialists in US often concentrate on only nineteenth century or early twentieth or such things as Officials or Revenues (US Revenues are not the world’s most plentiful; Mexico or Argentina or India would qualify for that, but they are by far the most avidly collected and expensive).
Specialty Catalogs: The Scott US specialized catalog is a comprehensive catalog. Its major fault (and the major problem with specializing in US stamps) is the overspecialization of its listings. For instance, in 1876, the United States reissued fifty or more postage stamps for the Centennial exposition. Scott would not have even listed these stamps if they were the product of any other nation. But, for US stamps, Scott gave these special issues full catalog status. Many of these stamps exist in quantities of less than fifty, catalog for tens of thousands of dollars, were produced on the same plates as the original, and are distinguishable only by experts (and not always even by them). Making them major catalog numbers makes completing US stamps a virtual impossibility (it’s only been done twice). Scott is similarly insane about grills and Washington-Franklins and is inconsistent about what is listed as varieties and as major numbers. US philately has suffered because the Scott specialized catalog is so mediocre and hard to use.
Specialty Albums: Scott makes a decent specialty album that comes both regular and hingeless. Mystic makes a more budget conscious album which has held up well. The Mercedes of US albums (as it is for so many countries) is Lighthouse. It’s very pricey but very beautiful, especially the hingeless version.
Expense: Collecting US stamps is very inexpensive after 1940 and very expensive before 1940
Availability of Material: As one of the world’s countries that is most keenly specialized in, most US stamps are readily availably, for a price. For most US collectors the search is not about finding a stamp but finding it in the grade and at a price that you want to pay.
Overall Collecting Grade: B+