Originally Posted by toddos
Using the tag browser, you can filter your book list to just those with that tag. I'm not sure how it gets any easier/better than that. Want to see all of your books about 18th century philosophy? Tag them, and then use the tag browser to filter by that tag.
One use of custom columns is to provide category-specific tagging. This helps keep the number of items in any category small, making the tags browser much easier to use. I can choose between the 6 items in my genre column much faster than choosing between the 30 items in the tags column.
Putting personal preference aside, the only strong argument I can make for not classifying tags into columns is that they can't be used for collections or in save templates. This annoys me to no end, which is unfortunate for me as I am the one who didn't finish the job when 0.7 was released. I will fix that, probably in August.
I should note that one use tags and still have category-specific tagging by using user-defined categories. Put the genre tags into one category, the subject tags into another, the work-flow tags into a third, etc. These are a bit harder to maintain because new tags would be added in two places, but the pay for that by providing a way to combine items from multiple categories.