As said above, difficult to answer, since it all depends on your needs and expectations.
What I did to decide the same thing is,
Do a tutorial of iMovie (lots of free tutorials out there) and then edit a project of your own.
Download the FCPX Demo (30 days unlimited), do another tutorial (ripple training has a free one that gets you going) and edit your own project again in FCPX. (don't forget to mess around with the features of FCPX)
After that you should be able to decide if iMovie will suffice.
I ended up buying FCPX, turns out I needed the color correction capabilities of it.