I bought the Sigma 120-300 Sport when it first came out, possibly one of the first to be released. I also got the 1.4 and 2x Sigma extenders.
IQ was more than good enough for me on it's own but was slightly disappointing with both converters, although I could live with them. The image below was standalone.
Biggest problems for me though were the weight and sluggish autofocus (although I never did get around to tweaking it on the dock which may have helped)
I am over 6ft and built like the proverbial BSH but it was tiresome on a long shoot when a monopod was impractical. Sometimes I needed to carry the 1DX/ lens combo in a rucksack instead of a proper bag and I found it just too cumbersome so I ended up selling it. Purely personal reasons I know.
Also, I think the lens hood is a poor design, fitting wise. It was so easy to think that it was fixed OK and then it fell off several times whilst walking. Never did get the hang of it.