I have since spoken to a few different people, all sing the praises of the lens but also say to get the most from it one really needs to use a tripod (lack of IS etc).

That is fine for someone who likes to stay put and not move around alot but alot of my photography is out in the field/bush and I move around ALOT so a tripod just is not practical. So now I am contemplating the 100-400mm as well.

Difficult decision and I may even continue to use my current lenses for a while longer...