Hadn't used HD 6.0.3 since I bought a MacBook Pro and moved on from OS 10.5.8.
Then, I came across this thread and was very excited by the prospect of using HD again, I use but loathe the current incarnations of iMovie.
Alas, my excitement was short lived. I was able to open HD in Yosemite, load clips, edit (titles, transitions, soundtracks) and then it would hang, file couldn't be reopened, and again, and again and again. So, so frustrating.
The reason I held off upgrading laptop and OS was a reluctance to give up iMovie HD, Eudora and Now Software's Contact and Calendar. I continues to mourn the loss of all four. I do misss 'em.