iWork 13 is more of a rewrite and reimagining of the iWork office suite than an upgrade to iWork '09. There are many functions missing from the new programs. This may or may not affect you depending on the kind of documents you have created.
For example: Pages 5 does do mail merge which is a serious loss for me. While Pages 5 allows you to create text boxes, it is no longer possible to link boxes to allow text to flow from one text box to another. This means my news letter template is unusable. My office mate, on the other hand, appears to be totally unaffected by the changes. He's found that a few documents in Pages needed a little re-formatting which isn't all that uncommon with an upgraded word processor.