I have a Sigma 150-600mm Sport, which is a hell of a lens, both in quality/features and uh...weight :D
It's totally hand-holdable, but reviews online make me wonder if everyone else has pipe cleaners for arms; it took a couple of weeks to get used to it but I don't have any trouble now. The image quality is fantastic and extremely competitive. The image-stabilisation works superbly, and the autofocus is very good - much better than their older lenses.
I mostly photograph birds and I'm extremely happy with the quality. All the recent bird photos I've uploaded are taken with it: https://www.flickr.com/photos/robotmay/50528664881/in/datepo...