In my experience as a Palo Alto immigration lawyer, the number one question is the obvious one: How long will my visa application take? The USCIS service centers provide bi-monthly tables that show how long different visa applications should take. The most recent table for the California Service Center is below.
http://www.scribd.com/doc/28190458/USCIS-CSC-Processing-Times-Table
USCIS CSC Processing Times Table
The column on the far left shows the actual Form number, and then the type of visa application. The far right column shows either a duration of months, or a specific date. If the time frame is shown as a duration of months, this mean that the time period is the USCIS' "goal" for that type of application, and that they are meeting the goal, for the most part. If the time frame is shown as a specific date, it means that the USCIS is not meeting its "goal" for that type of application, and that the date shown is the submission date of the last application adjudicated.
For example, the chart above shows that an I-129 visa application for an O-1, Extraordinary ability nonimmigrant, is taking approximately two months to adjudicate. For I-90 applications to replace a Permanent Resident Card, the California Service Center last adjudicated an application submitted August 17, 2009.
The information is useful for two reasons: First, I-129 nonimmigrant work visa applicants can choose to submit their application via "Premium Processing". For an extra $1000 filing fee, a visa applicant can submit an additional form and their application is supposed to be adjudicated within fifteen days. (However, if the USCIS sends out a Request for Evidence, or a Notice of Intent to Deny, the fifteen day time period is suspended because the applicant must submit additional evidence.) If a visa applicant sees that it is only taking two months to adjudicate their type of visa application, they may choose to forgo "Premium Processing", and just wait two months. Of course, if the table shows that their application type is taking nine months to process, then an extra $1000 might be worth it so that the beneficiary can begin working in the requested visa status.
Second, if an applicant sees that their application has been pending beyond the date on the table, they can take specific steps with the USCIS to check on the status of their application. Generally, the USCIS will only check into the pending status of an application if it is beyond the published processing time.
Despite the usefulness of this information, I find these tables difficult to locate on the USCIS website. Aside from bookmarking this webpage, I find it easiest to check processing times through the USCIS Processing Times Chart link on my website.