since you have tamron 17-50, why not considering sigma 50-150mm f2.8? Combine with your tamron, you'll get a good pair of fast lens.
I have cz1680, and I must say this is a good lens that I will never release at least I got better lens like cz2470 (which I can say far away from my dream).
Yes, have considered to get the F2.8 double team but as my wife might be using the cam to take pictures of me and my son (since she can't swim....), so the weight of the lens is a major consideration.
We tried using tam1750 once at the poolside and she practically has to stand next to the pool in order to take some closeups....and kinda dangerous to do that...(dunno whether i am worried for her or my cam :bsmilie

That's why shortlisted 16105, 1680 and 18250 and thinking of bringing my son to animal farms, so these one-lens option is quite useful esp when u have to lug around kids stuff.
Although i can still hear the sound of cz calling for me, but wanted to make sure the "build quality" issues is not so bad afterall.
Probably i think since many paid so much for the lens, their expectations could be set too high for the lens which is actually meant to be affordable CZ for normal consumers like us?