How much disk space do you have? iMovie 10 is likely to eat it ALL.
One thing where it is better than iMovie 9.0.9 though, I've found, is in the stabilisation. But it takes a while to do that, longer perhaps than you see the spinning thing instead of the checkbox. It also needs a lot of space and it doesn't seem to complain when it runs out; it just quietly fails to do what you asked.
I tried it out and decided to stick to iMovie 9.0.9 for most of what I do, but I did reinstall on a user account on my MacBook that I don't usually use for video, because there were a couple of things where v10 seemed better than v9. For me, those are the image stabilisation, and the fact that v10 will repair the synch problem in videos taken on my Samsung Galaxy S2 when I had Android v3 on it. On the account where I'm still using iMovie v10, I have not quite 30 minutes of video in the Movies folder. Here are the figures:
What was there already, created using iMovie 9.0.9
iMovie Events: 4.33 GB on disk
iMovie Projects 54.7 MB on disk
What resulted from (1) conversion process and (2) appying shake compensation to a 20-min video in v10:
iMovie Library: 33.63 GB
At least the conversion process is not destructive. You can go back by deleting the iMovie Library and iMovie Theater files, and by removing the UpdatedToiMovie10 marker files from the old Events and Projects folders.
The old stabilisation process would crop and crop to keep the steady picture within a stable frame, but you could lose an awful lot of visual information. That's why the slider asks you how much zoom you're prepared to tolerate.
The new stabilisation process asks you what percentage of stabilization you want to achieve, zero to 100. At a certain level, and (as far as I can see) depending on how shaky your video is, it cuts over from zooming in on a smaller area to a kind of "dancing frame" effect, where you see the edges of your video within a black frame that dances around. Points on the picture are stable, and you do see more of the image, though the edges drop out of sight behind the black frame when the shake sends it that way.
I quite like the dancing frame, which is why I persevered, but what annoys me is that iMovie v10 makes its own sweet mind up whether to use the crop or the dancing frame method, and it does that clip by clip. I want to control how my video looks in its entirety, so I find it intensely frustrating that it's not an option I can control.
The 20-minute video where I'm re-doing the stabilization is of a helicopter trip to the Grand Canyon, for which I used my trusty Samsung S2 and which you can guess was quite shaky in places.
I'm rendering that now, and will upload to YouTube after it's done. In case anyone is interested in comparison, I posted the video as stabilised by iMovie v9 (it refused to do some clips), and allowed YouTube to make a copy of that version adding their own picture stabilization. I've left both for comparison, and some time in the next few days I'll post the v10 result.
https://www.youtube.com/user/JimInRoses/videos
