I guess the things I like about Thomsons are:

1. The finish. Thomsons, in my opinion, just look nicer than most posts. Glossy, yet manly. Plus, cosmetically they hold up better than most posts I've used. A heavily used Thomson looks nearly as nice as a brand-new Thomson (this is especially beneficial when you're buying used to save a few bucks).

2. Adjustment. True, there are other posts out there that offer very fine adjustment, but again I have not personally used a post quite as good in this respect. Plus, there are numbered markings for adjusting the angle, so if you remember what number you like it set at then you can quickly dial in your saddle angle without a level or trial-and-error.

Alas, I no longer use Thomson posts on most of my bikes because I need more than the 16mm of setback their setback posts provide. And sliding the saddle far enough backward is not an alternative option, because of the larger clamp.