We deploy a lot of WordPress sites for clients. Tags are a great new tool, beyond the basic blog use. For example, clients can tag their pages (requires a plugin), allowing us to display related posts in the sidebar. (Useful note, if you’re doing this: get_the_posts() does accept a post ID, and does work outside of the loop, documentation notwithstanding.)
Tags are still a bit new by WordPress standards, so you may have to deal with some bugs. In particular, you can’t search for posts that belong to category A and have tag B (more information in WordPress trac). Luckily for us, the bug was already fixed in August. Unfortunately, the fix hasn’t been incorporated into release, and somehow this bug has been marked as an “enhancement”.
We all know what “enhancement” means in a bug database, so I’m uploading a fixed query.php for WordPress 2.7. So far, we haven’t noticed any issues with the fix, and we’re enjoying the new functionality.