I'm core level certififed in Word and Excel.  In my opinion it a 50/50 toss as to whether it helps.  Most managers I have interviewed with have never heard of the certification, unless they are in the IT department.  On the other hand large corporations with HR departments who are "in the know" do know what the certification means and may even reimburse for testing fees.  Will I go for master level certs - probably not, however I would like to add Powerpoint certification on my resume.
As for difficult, I tried Word cold with out studying and failed.  I bought a Sybex textbook and passed the second time.  Excel was a easier for me and yet I would have said I am better with Word.  I recommend getting the textbook that has an appendix with the test topics to know all bases are covered.  I would also question the testing facility as to the level of the questions.  For example right click options may not be counted as a correct answer (and F1 isn't available!).  As we all know there are many ways to complete the same task in word, but only one may be the correct answer on the test not the end result.