It's a question that gets debated endlessly but one that everyone has to answer for themselves. It depends on how many bikes and wheels you have, how easy/hard you feel swapping components is, and how much money you have to spend on a PM.
I recommend reading the Coggan and Allen power meter book now. If you can't get through the book, then reconsider buying a power meter. You won't be getting full value out of it. Lots of people go fast without one.