Note that this update is huge! If you're running Mac OS X 10.5.5, the download is 372MB. If you're running Mac OS X 10.5.0 through 10.5.4, it's a whopping 688MB. Expect the download to take a while.
Here are the more significant improvements offered by this update, but there are more that I've not listed. You can read the entire list here.
Address Book  Improves reliability of Address Book syncing with iPhone and other devices and applications.
Graphics  Includes graphics improvements for iChat, Cover Flow, Aperture, and iTunes.  Includes fixes for possible graphics distortion issues with certain ATI graphics cards.
Mail  Fixes an issue that could cause messages identified as junk to remain in the inbox.  Addresses an issue that could prevent Mail from quitting.  Improves reliability when printing PDF attachments.
MobileMe  Contacts, calendars, and bookmarks on a Mac automatically sync within a minute of the change being made on the computer, another device, or the web at me.com.
Parental Controls  Addresses an issue in which a parentally-controlled account could be unable to access the iTunes Store.  Includes general fixes for time limits.  Resolves an issue that prevented adding allowed websites from Safari via drag and drop.
Time Machine  Fixes issues that could cause Time Machine to state the backup volume could not be found.  Improves Time Machine reliability with Time Capsule.
Safari  Improves compatibility with web proxy servers.
General  Adds a Trackpad System Preference pane for portable Macs.
There are also quite a few security updates, which you can read about here.
This update is recommended for all "Leopard" users, but of course, in a business environment you should test it thoroughly before deploying across your company.