I use Google Reader to read blog feeds, but I never noticed good tagging functionality. I mean, I sort the blog I follow into a few folders, and it calls those folders "tags" sometimes, but while I have only a handful blog categories I sort feeds into, I have about 3500 different delicious tags I assign to my bookmarks. The interface doesn't seem suitable for this use. I mean, I don't see how a sidebar with 3500 folders would work out.