I use FrameByFrame, and overall I think it's decent for a free program. I wouldn't necessarily suggest switching.
After an update a year or two ago, Quicktime now no longer loads certain types of .mov or .mp4, which is odd. I have literally no idea why. It's normally fine for me, as Quicktime automatically tries to convert it into a file it can read (though this does compress it somewhat). That said, I just checked and it doesn't open in my iMovie either (but that hasn't been a problem for me, since I don't use iMovie, and would honestly suggest that you find a better piece of editing software since iMovie is pretty limited).
I'll have a think and a look around and let you know if I come up with anything.
"Nothing goes down 'less I'm involved. No nuggets. No onion rings. No nothin'. A cheeseburger gets sold in the park, I want in! You got fat while we starved on the streets...now it's my turn!" -Harley Morenstein