That lens of NHL's weighs 2.6kg without the TCs and is 300mm long; I personally wouldn't take it as a gift even though optically it is a fantastic lens.
... It all depends on how much one values the faster aperture f/2.8 (same size/weight as an EF 300 f/2.8L ):
Wildlife are most active during the early or late hours - When they are mostly sleeping or standstill at midday I guess you can use IS over the two extra f-stops!
A compromise would be the Sigma 100-300 f/4 EX
Split the weight right in the middle (and it's MTF is higher than the EF-300 f/4L), shorter and still good enough with a 1.4x for a full 420mm @ f/5.6! :-)
The Sigma 120-300 f/2.8. Okfair enough. Only problem being the loss of focal length between 85 and 120. I wouldnt be happy with that personally.
This is why I mentioned the Sigma 18-125 :idea:
Check the posted link - It's doing quite well in low-light... http://www.fredmiranda.com/forum/topic2/143791