VACUUM FULL is not recommended for PostgreSQL versions prior to 9.0, except in extreme circumstances.
VACUUM FULL should only be used if large amounts of data have been updated or deleted in a table, in order to release space back to the operating system. In cases where the space can be expected to be used by the table in the future, a regular VACUUM should be used instead.
VACUUM FULL will take exclusive locks in the database, preventing all other activity on the table(s) being VACUUMed, and as such will have a very large impact on other users of the database while it runs.
After a table or database has been VACUUM FULL'ed, it is recommended to REINDEX all indexes on the table as well.