I once bought a $1200 Sigma 120-400mmm based on my high opinion of Sigma. In the end it cost me 16 hours driving, hundreds of dollars in gas, and I ended up with a DA*60-250, that is actually a great lens. Probably better than anything Sigma makes in it's price range.
Quote: it's all about the optics. Plus Sigma wins rather massively when it comes to build quality and reliability
That's a pretty dangerous opinion. It could cost people a pile of time and money. It cost me a lot of time and money. I don't entertain that opinion much anymore. At least not quietly. I buy Sigma when Pentax and Tamron don't make a similar product.
I am thinking a Sigma 70-200 2.8 would be an good investment, just because for me that would be an action lens and it' is said to be very fast focussing compared to others in that range. I've ordered the 8-16, apparently they did a great job on it, and the 300 and 500 primes are impressive. Apart from those four, I wouldn't recommend any of them.
When they don't compromise they produce great lenses. Their compromise lenses are as bad as anyone else's compromise lenses.
Apart from those four, my opinion would be, don't buy one unless you have to.