I've just issued a bug report and patch for a problem with the Search Query class. It's not a huge bug but it prevented me from doing something I needed to do: add some restrictions to a Search Query.
There are two ways of modifying queries in Drupal 7, you can either use a generic "hook_query_alter()" or use "hook_query_TAG_alter()". Tags are a way of telling other modules what a query is about. For example you can have a tag "node_access" which allows modules to add additional restrictions on the query, in this case the Node module connects the query to node_grants.
The Search Query adds a "search_node" or "search_user" tag, unfortunately it adds the tag after the hooks have been called - which is as useful as a chocolate teapot. So I added the patch here: http://drupal.org/node/1146564 which simply moves the place where the tags are added.
Another thing to watch out for: the generic hook_query_alter() is not called on queries that have no tags. This is an efficiency thing as it was discovered early on that this hook was called a lot for no good reason during bootstrap.