HOWTO list

UNANSWERED: How to tell WSUS (or BITS) to prioritize download for certain update

how to find the current actual path of wsus installation and its update content
HKLM\Software\Microsoft\Update Services\Server\Setup in the value "ContentDir" (source)

Move store folder to new location :
Changing the Location where You Store Update Files Locally

Integrate 3rd party updates with WSUS:
Windows and 3rd Party software update automation and tools
Two of them are open-source (one is LUP mentioned by Brian), and two of them are commercial.
(From wiki: update file must be MSI, MSP, or EXE file.)

WSUS Step by Step

Monitor current BITS download:
BITS GUI Admin
BitsMonitor (seems not suitable for monitoring WSUS updates download; display is empty)

Checking if certain update has been downloaded:

WSUS synchronizes the metadata for the updates but doesn't download the update files until you approve them. You can see whether they have been approved by looking at the Approval column in the WSUS console when in the updates view. If the update has been approved, you can check the download status for the update by going to the updates view, right-click one of the column headers to get a list of available columns and select File Status if it's not already selected, and then the File Status column will display an icon. If you place the mouse cursor over the icon in the File Status column it will give you the download status for the update. For example, for approved updates it will give you the status of "Ready for installation" meaning the update file is downloaded, "The update is downloading", and for unapproved updates it would give something like "Ready for installation (files not downloaded)", etc.

Source:
http://social.technet.microsoft.com/Forums/en-US/configmgrsum/thread/f380242d-
ea37-4296-b1cd-2474e0b32620/
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Express Installation Files (EIF): use or not use?

Options \ Update Files and Languages

Express Installation Files are specifically designed for use where clients
are supported across restricted bandwidth WAN links.

The savings on your internal bandwidth will be significant where larger
updates are concerned, as only those bits /needed/ by the client are
actually transported across the wire, as opposed to the entire update
package.

However, the cost to your Internet bandwidth may be as much as 10x higher,
as the typical EIF package is about 10x larger than the normal update
package.

If you have 10 clients on slow-speed WAN links, then you're probably ahead
of break-even, since you'd be downloading 10x the content, but sending less
than 1x the normal update package to each of those clients.

If you have less than 10 clients, it's probably not worth the disk storage
on the WSUS server to maintain the larger download files.

If you have more than 10 clients on remote slow-speed WAN connections,
Express Installation Files are definitely recommended.

Note: Not all updates are available with EIF packages

Source:
www.winvistatips.com/wsus-express-installation-files-t742408.html
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Migrating WSUS to a new server without downloading all the updates

Source:
http://blogs.technet.com/b/industry_insiders/archive/2007/03/15/migrating-wsus-to-a-new-server-without-downloading-all-the-updates.aspx
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