Using tags, or as it's know on the net - folksonomies - is very helpful in locating information. I am constantly using tagging on my own personal computer for example. In MS Vista, you can easily tag files and folders with your own words, so you can find them very fast as you need them. This works the same way out on the 'net, using sites that have organized their content using tags.
I'm even using tags to find other blogs in the 23-things project that use terms like 'thing' and 'resa'.
A disadvantage of using tags is that your description of an item might be totally different than others, and might not add to the larger group of users trying to find information. Additionally, there is a lack of precision when one describes something with a one-word tags.
I suppose if Melvil Dewey could see what was going on, he would not be amused.