Paul,
I tried the 120-300mm on an EOS 3 today. I'm gonna buy it. It feels solid, the range is great, and the autofocus is really quick. It's hard to know if it's as fast as my Canon 300f/4L, but I'm willing to compromise a bit for that zoom range and a 2.8 aperture. Know what I'm sayin?
I'll tease the Canon reps about that 600mm tomorrow.