Depending on the build of Windows Mobile 5.0 installed on your smart devices the upgrade will be free for some users while others will have to pay $49.95 for the upgrade kit. (Yikes!) Before purchasing the upgrade, I strongly suggest that you check with your smart device vendor to see if they have any Windows Mobile upgrades available. You might be able to upgrade your install of Windows Mobile and then qualify for the free Office Mobile upgrade.
On the Microsoft Office Mobile Upgrade or Purchase page, there is a simple series of questions that helps you determine if your device qualifies for a free upgrade or not. Microsoft was nice enough to publish a Office Mobile 6.1 FAQ so you can learn about what is included in the latest version of Office for Windows Mobile devices before pluncking down the cash for the upgrade. Upgrades can be purchased from Handango, and volume licenses are available if you have a fleet of Windows Mobile 5 devices to upgrade.
I see this upgrade being important for users who find themselves with a Windows Mobile 5 device with no vendor provided upgrade to Windows Mobile 6, and who need support for Microsoft Office 2007 files. If you are a non-corporate user who doesn't need the latest and the greatest, I would skip the upgrade until you need to purchase it. $50 is a lot of money to gain support for Office 2007. DataViz Documents To Go 10, which added support for Office 2007 on the Palm OS platform, only ran customers $29.95.