As with any tool the best use can be obtained by knowing the capabilities and limitations of the tool. The Metamorph index allows for rapid location of records containing one or more keywords. The Metamorph index also takes care of some of the set logic.
The following should be noted when generating queries. The most important point is the choice of keywords. If a keyword is chosen that occurs in many files, then the index will have to do more work to keep track of all the files possibly containing that word. A good general rule of thumb is "The longer the word, the faster the search".
Also, neither type of Metamorph index is useful for special pattern matchers (REX, XPM, NPM) as these terms cannot be indexed. If other indexable terms are present in the query, the index will be used with them to narrow the result list, but a post-search or possibly even a complete linear scan of the table may be needed to resolve special pattern matchers.