And let the downloading begin!

517.3MB in my software update panel:

The 10.6.5 Update is recommended for all users running Mac OS X Snow Leopard and includes general operating system fixes that enhance the stability, compatibility, and security of your Mac, including fixes that:
improve reliability with Microsoft Exchange servers
address performance of some image-processing operations in iPhoto and Aperture
address stability and performance of graphics applications and games
resolve a delay between print jobs
address a printing issue for some HP printers connected to an AirPort Extreme
resolve an issue when dragging contacts from Address Book to iCal
address an issue where dragging an item from a stack causes the Dock to not automatically hide
resolve an issue with Wikipedia information not displaying correctly in Dictionary
improve performance of MainStage on certain Mac systems
resolve spacing issues with OpenType fonts
improve reliability with some Bluetooth braille displays
resolve a VoiceOver issue when browsing some web sites with Safari 5
For detailed information on this update, please visit this website: http://support.apple.com/kb/HT4250.
For information on the security content of this update, please visit: http://support.apple.com/kb/HT1222.

On appleinsider, they list the fixes:

The full list of general fixes and improvements in Mac OS X 10.6.5, according to Apple, includes:
Improves reliability with Microsoft Exchange servers.
Addresses performance of some image-processing operations in iPhoto and Aperture.
Addresses stability and performance of graphics applications and games.
Resolves a delay between print jobs.
Addresses a printing issue for some HP printers connected to an AirPort Extreme.
Resolves an issue when dragging contacts from Address Book to iCal.
Addresses an issue in which dragging an item from a stack causes the Dock to not automatically hide.
Resolves an issue in which Wikipedia information may not display correctly in Dictionary.
Improves performance of MainStage on certain Macs.
Resolves spacing issues with OpenType fonts.
Improves reliability with some Bluetooth braille displays.
Resolves a VoiceOver issue when browsing some websites with Safari 5.
Improves Bluetooth pairing with Magic Trackpad.
Resolves performance issues with third-party displays that use InstaPort technology.
Add SSL support for transferring files with iDisk.
Resolves an issue when opening 4-up Photo Booth pictures in Preview.
Addresses keyboard responsiveness issues in the Dock when Spaces is turned on.
Resolves an issue syncing Address Book with Google.
Fixes an issue when replying to a Mail message sent by a person whose name contains certain characters such as é or ü.
Improves performance for users bound to an Active Directory domain.
Improves reliability of Ethernet connections.
Systems with a Mac Pro RAID Card (Early 2009) installed can now be put to sleep.
Improves reliability of fibre channel connections, resolving a potential Xsan volume availability issue.
Adds RAW image compatibility for additional digital cameras.

Got it installed here, no problems.

No problem so far for me. Mine was 497 MB - a whopper!

The combo is nearly a gigabyte .

One new feature I notice is iPhone/iPad like. When I two finger scroll with my trackpad on my MacBook Pro, the page keeps flow scrolling. Versus before when you let go the scrolling stops on the spot. Now scrolling works like it does on the iPhone/iPad.