Sigma has definitely stepped up their game recently.
I am currently on the fence to buy the 18-35 1.8, what holds me back is the hope for some further price drops when the early adopters have been fed.
The 120-300 2.8 OS is definitely something I would love to have, both for focal length (my longest serious lens is the 70-200 F4 IS, not counting the Tamron 28-300 VC, which has been setting on the shelf for a while now...
) and for speed, but unfortunately at 3+kg it is a tad too heavy and at 3k€ also too pricey for a hobbyist.
What really is missing is a serious contender for the Canon 100-400, i.e., a moderate zoom-range, only a f5.6 aperture at the long end, but weight < 1500 grams and price <1500€. And of course this lens should continue the trend of Sigma's latest offerings - awesome image quality already wide open.
Neither the old 120-400 OS, the 150-500 OS, nor the 50-500 OS are good enough in my book to seriously challenge the ancient Canon 100-400. If Canon finally announces a successor, it will probably be 20% better and cost twice as much, which finally moves it out of my hobbyist budget. That will definitely leave a huge gap for Sigma to fill.