I upgraded to FCPX in May when I bought my new iMac. Up until then I had been using various versions of iMovie and just felt that FCPX would help making movies more flexible, with many more option than are possible with iMovie.
FCPX is amazing. Its very responsive and the colour corrections and grading (with additional plugins I bought) make the process of creating the finished project very easy and fast. I am currently working on an edit of a wedding video made in 1997 from two VHS camcorders, trying to cut between the two cameras (they filmed the ceremony from two angles). Finally I tried the multi-cam feature in FCPX (I needed a couple of tutorials to understand how it worked) and WOW, The two camera were synced up on the audio track and I was able to cut easily between the two, Took me 30 minutes or so to get the rough edit. Yes; this is analogue tape from 1997! I just love it!
The plugins I bought are very responsive, I just have to move the mouse over the plugin to instantly see the effect on the clip. No rendering time required! I have more flexibility (than iMovie) with titles and their placement etc and the transform tool allows me to shrink and move clips around the frame (using keyframes) very easily.
The only issue Ive been fighting with is that I have been unable to successfully import all the iMovie projects into FCPX for some reason. Some worked. Some didnt. A visit with a Genius at my local Apple shop did not resolve the issue (It is a software issue, apparently). There is no other way to get these projects into FCPX, other than exporting the movie from iMovie, importing the movie into FCPX and cutting the scenes into individual clips (for grading / letter boxing. This is the only issue ive had, so far. Too bad really. Obviously this problem will not exist if you dont have files to transfer.
With knowledge of iMovie, the learning curve is easy. Note however that there are lots of hidden features in FCPX that you will need to uncover and learn - but this is all part of the fun! Highly recommended for anyone who has outgrown iMovie, or needs to do some serious image editing. Lots od plugins available!