If u look at prices of original manufacturer equivalent lens like the 17-55mm F/2.8, it's abt $1.6k (or more I'm not too sure). A tamron 17-50 F/2.8 cost only abt 1/3 the price or less for a local copy. Relatively, a $500+ lens is consider quite cheap as to compare to other lenses. Do note, when I say this lens is cheap, I'm in no way infering that this lens quality is relative to it's price.
With regards to lens warranty, it's all up to ones preference. Like other electronics, lens can fail. I'd a spoilt tamron lens with local warranty before, brought to JEL and got it fixed for free. But what did that actually cost me? One day leave, $9 for lens deliver to my home (or else i've to make another trip down again) and 3 working days. It's not much actually, but to think back, if tamron lenses do break down easily, does that means I've to take leave often to fixed? (JEL is at Changi Way North btw and they operate only during weekdays and working hours) if it doesn't spoilt easily, why am I paying $200 extra for something I've no use.
I've stated my reasoning, it's all up TS to decide.