I started collecting calculators without really thinking about them as collectables. I am just one of those typical engineers that's just got to have the best equipment. When I came upon the IACC page, I realized that it was hobby that others shared and I immediately got more serious about it. My interactions with the others that share this hobby have been some of the most enjoyable and rewarding times in my life. Collecting calculators is a nice hobby and calculator collectors are nice people.
There is no concerted effort to focus in one area. I collect the calculators that I like. There have been some questions about the limited number of TI calculators on the site. The answer to this is that I was always an HP guy. In 1972, I waited in the rain outside an HP field office to pick up my HP-35. I was a young college student and that machine shared some the happiest times of my life. TI calculators have never interested me, and that's why I don't collect many of them.
The later HP machines 2x (Woodstock) and 3x machines aren't of great interest either. I saw HP making a concerted effort to lower the cost and quality of the machines. If you look at the number of 35's and 45's that still work (with the possible exception of an on-off switch) and then compare that to the HP 2x series, you will see that the quality did slip. By the advent of the 3x machines, it was terrible.
The HP41 was a different story and to this day, it is a favorite machine. It's easy to program, had massive IO capabilities, lot's of peripherals. It's a great machine today as well.
If you hit the "Monroe" button, you'll see a wide range of scientific machines aimed at the scientist and engineer. I used to think of Monroe as the Electrolux of Calculators: beautifully made, slightly behind the time, and sold by travelling salesmen. Monroe specialized in desktops because the technology hadn't reached the point that we could get one in the pocket. The 20-40 series (large pocket) represent the height of quality in pocket machines. The panaplex display was easily readable at all angles and in very high ambient light. The 1920 and 1930 models had a beautiful, distinctive style and were very functional desktop machines. My favorite machine is the Monroe 326G with tape drive. This machine saved me from the computer center (mainframe with cards.....Gads, I am old). The tape drive could be used to write data or programs. A continuous loop tape was available and I was able to write a fourier transform program (1976) to do some calculations for my thesis. It would take about 1 hr to do the work, but it sure beat trudging to the computer center in the snow.