I have the 300 f2.8 I IS and regular shoot it with a 1.4x or 2X TC hand held. Its a bit heavy if you have to walk a couple of miles with this combo, but it is doable for me. The attachment is a portrait of a black bear in Yellowstone at 600mm, 1/200 sec, handheld. The iS is great. So the answer is it depends on your level of fitness, how far you need to walk, what tele photos you have used in the past, etc. In general, you will find many people amateur like me or pros who say this lens is very handholdable.
I hope this is taken as constructive opinion, but the image of the bear, is not sharp at all.