I think it depends on your personal style, and how important it is for you to do work when you are away from your desk.
I bought a Nokia E61i, in part because it included features like PDF viewing, and the ability to open and edit Word and Excel documents.
I would say I have used the Word and Excel features maybe two or three times in two years. The screen and keyboard are just too small to be productive.
For communicating - email, instant messaging, Facebook updates, etc., these devices are great. For typical office productivity applications, I think you have to REALLY need it in order to find it useful.