Just finished August's Breen's Bungalow in version 4.0.1 of iMovie. Couple of comments:
Comment 1: Apple's claims that this version is more responsive than 4.0 is true -- but it's not true enough for my tastes. Dragging items around the timeline in complex projects is much faster than it was previously. A sincere pat on the back for that one.
But saving is still an exercise in patience. On my 933MHz Power Mac G4, iMovie took well over a minute to save once I was well into a piece that included 60+ clips. Besides this being an annoyance, it's likely to cause people to save less often and, as I discovered, this is dangerous.
After performing a dozen or so edits, my Mac crashed with a kernel panic. I had to redo the edits after rebooting the Mac and took pains to save after every edit (and tried to be patient as the beachball cursor spun and spun and spun and spun).
Comment 2: This version of iMovie remains slow about recording audio. More than a couple of times I recorded a voiceover only to discover that the first bit of narration was missing because it took iMovie so long to begin recording. When rerecording these bits I waited until I saw the sound file appear in the timeline before I began speaking.
Comment 3: Speaking of audio, the preference to Show Audio Track Waveforms is potentially a wonderful thing. With this option enabled you can see where your audio bits begin and end -- allowing you to easily trim the silent portions at either end.
Regrettably, these waveform displays aren't always accurate. I trimmed a couple of files only to discover that I'd cut out portions of the narration. Dragging the file out to the file's entire length and playing them revealed that audio existed in some places where no waveform appeared. My advice, therefore, is that if you use waveform display, you preview your files before trimming them.