I'm using the newest WordPress and Simple Tags. WordPress 2.7 provides a 'Tags' widget in the right sidebar of the writting page. I can add some tags through it but the tags can't be saved. However, Simple Tags provides a text area 'Advanced Tags' on the bottom of the editor. I can only add tags with it.
If I disable Simple Tags, the 'Advanced Tags' disappears and the 'Tags' widget in the sidebar can work well. i.e. I can add tags with it and save them. If Simple Tags is enabled, the 'Tags' widget can not save tags.