iMovie is quite good! I can say from experience that if you feel comfortable with it, you wouldn't have a hard time transitioning to FCPX if you wanted to. They're built on essentially the same interface, so it immediately feels very familiar to work with and you can do the basics with basically no learning curve at all. There are a few things I've had to google, but not too much. And there are neat little upgrades that save a lot of time. Like for slow mo, in iMovie you have to specify a percentage speed and then it adjusts the clip. For me this often meant making repeated tiny adjustments to get the clip to fit into the exact space that I wanted. With FCPX you do Cmd+R and then you can literally just drag the clip to shrink or expand until it's the right size. Handy little things like that!