for you, and your repliers:

this data is already available as a downloadable file. Check out ftp://ftp.census.gov/ It's CSV, and it's kinda ugly-looking, but it's available.

The 'full' data set is ...rather huge. The full data set for any particular report is rather huge.

The advantage of this API, I expect, is it allows people to make quick, machine readable queries for useful subsets of the data. If you want a JavaScript mashup that tells you the number of people who live on your block, use this. If you want to know the population of every block everywhere for data-crunching purposes, just download the CSV.