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 ï¿¼ Improves reliability of Address Book syncing with iPhone and other devices and applications.
Graphics ï¿¼ Includes graphics improvements for iChat, Cover Flow, Aperture, and iTunes. ï¿¼ Includes fixes for possible graphics distortion issues with certain ATI graphics cards.
Mail ï¿¼ Fixes an issue that could cause messages identified as junk to remain in the inbox. ï¿¼ Addresses an issue that could prevent Mail from quitting. ï¿¼ Improves reliability when printing PDF attachments.
MobileMe ï¿¼ 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 ï¿¼ Addresses an issue in which a parentally-controlled account could be unable to access the iTunes Store. ï¿¼ Includes general fixes for time limits. ï¿¼ Resolves an issue that prevented adding allowed websites from Safari via drag and drop.
Time Machine ï¿¼ Fixes issues that could cause Time Machine to state the backup volume could not be found. ï¿¼ Improves Time Machine reliability with Time Capsule.
Safari ï¿¼ Improves compatibility with web proxy servers.
General ï¿¼ 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.