The New lens has much faster AF, and is more familiar to handle compared to my version one. I never had a dust issue with any of my lenses, even if they get a little in them, thats not a factor in IQ.
It works very well with TC's, I forgot and left my 1.4X TC on the lens when I first got it, and used it 2 weeks that way never noticing any IQ issues.
For sharpness, its about the same, but, the build is better which likely means less copy to copy variation.
I really do not notice the improved IS, probably because I'm used to using the highest possible shutter speed.
With refurbs going on sale occasionally at Canon, it becomes a bargain. I can't personally compare it to the Sigma or Tamron 600mm models, there are lots of reviews though.