I've had two 300/2.8 L lenses (non-IS) over the past twenty-one years. They've earned their place as my all-time favorite lens. You could say I've grown very attached to mine, though I suppose not as much as my 1DS (III), which, in terms of sharpness, contrast and "look" (in part, the bokeh), I must say is a most amamazing combination!
While I've used the IS version, I've never owned one, as it doesn't seem to give much if any improvement in my specialty. You see, I frequently shoot hand-held from moving boats and often the conditions are such that the motion is too great even for the stabilization. In times like that, one needs to rely on good technique. When that's not good enough, it becomes time to use a gyro.
This lens works well with teleconverters, especially the 1.4 x. I'll frequently attach one when I need some extra reach and I've found find there's minimal loss in IQ. The 2x is best reserved for those situations when you can't get any closer and absolutely need that larger image size, though the Mk III version I have now gives pretty good quality. It probably only disappoints because the lens by itself and with the 1.4 is so good.
In short, I have NO reservations about you getting one of these lenses, unless you HAVE to have IS. In terms of getting the most bang for your buck, this lens should be at the top of your list!