Okay, should have given a little more detail: I've had the 100-400 and 50-500OS, of the two the 50-500 was the better, almost as good IQ, better IS\OS and improved range - the 150-500 isn't that much worse but I needed the money at the time so downgraded.
I'm not a fan of lens swapping, which also includes adding\removing TCs.
The reason for all this is possibly due to what and how I shoot - I get bored shooting alone so almost always double up the outings with the family - two young kids and a wife aren't that keen on extended stops 
I often shoot the kids while I'm out but if I have a long lens at the time I will make 90% of my shots of the birds, squirrels, dear - whatever it might be - thats where the draw of zoom came in, however the sluggish power up time on the 150-500 had made me miss more shots than I care to count - so for example I'm out, see a swan taking off, raise the camera and wait for the motor AF to start up (camera already on of course), once the AF starts going and gets round to focusing all I get it the beasts backside... annoying to say the least and its happened so often. I have of late taken to half pressing the shutter every now and again just to keep it awake which has helped alot.
So as you can imagine this snapshot approach also rules out monopods and tripods.
It may also be worth noting that if I'm out taking real images of the family I take the 135mm f2 - nothing beats that for really nice shots.
The loss of the 100mm was mentioned - I have a feeling (and have read it somewhere too
) that the Sigma 500's don't quite reach 500, more 450 or so, so presuming that Canon are true to their word and really are 400 then it wont be that bad - its more the short end I'd miss.
