Yes Susan, I am MOS certified and it was worth the effort.

I have been for several interviews over the years since achieving this qualification and most have never heard of it - that makes it a great thing in my book!  

It was not necessarily a challenge as I had reached advanced level in all aspects of the MS Office suite, however it was a goal I set for myself as regards personal growth and development and something I wanted to add to my CV.

As for tips, it depends how you are learning it.  I attended night class for 15 weeks and passed 3 exams in that time, for the remaining 2 I studied myself using the Microsoft books and cd rom.  However if you are using it every day you should be more than halfway there.  You can also purchase tests to do, also available on cd rom.