HISTPOP is one of my favourite sites, and at first glance seems to have to great appeal to genealogists; the home page even states ‘Note to genealogists and others tracing individuals: This site only contains a very small number of reproductions of original census enumerators' books for illustrative purposes.’ This is true, but of course a good genealogist is interested in more than just lists of names.
The site is provides background information on population statistics, through the work of the General Register Office. It includes the Annual Reports of the Registrar General for England and Wales, and the Census Reports up to 1931. There are scanned examples of documents held in The National Archives on census preparation, for example, and there is quite a lot of material for Scotland and Ireland too.
If you click on the 'Browse' tab there are several sections to choose from that explain what you can get from the site. The original source material is great, but I'd recommend that you look at the essays, written by Edward Higgs and Matthew who really know their stuff.
Or you can use the Search function to explore the site; try putting in the name of a disease or an unusual occupation and see how often it appears. Have fun