Ancestry.com is always one of the first places I go to look for records for someone who has done little or no family history work. When Ancestry.com has the scanned records, such as the U.S. Census and World War I Draft Records, the site is extremely helpful. The images often provide much more information than the indexes. However, in some instances, when there are only indexes, the records are sometimes not so useful. Without viewing the original documents to compare to the record transcribed into Ancestry.com, I hesitate to rely on the accuracy of the transcribers. I have probably spent more time on Ancestry.com than any other online database, but I still find that there is no excuse for viewing original records that have yet to be incorporated into Ancestry.com's huge resource.
I also do a significant amount of work in Spanish. I find that Ancestry.com has few records that assist in basic Spanish research. For example the 1790 California Census is available only as an index, Ancestry.com refers users to the microfilm. Quoting Ancestry.com
All available census schedules, from 1790 to 1920, have been microfilmed and are available at the National Archives in Washington, D.C., at the National Archives' regional archives in twelve states, at the LDS Family History Library and LDS family history centers throughout North America, at many large libraries, and through microfilm lending companies. Some state and local agencies may have census schedules only for the state or area served.8 billion records is very impressive. At some point, however, I think they may need to beef up their search engine to give a broader finding capability. All said and done, Ancestry.com is a modern wonder of the computer world.