My reason is that I've sprung for a Tamron 200-500, and it was pretty poor. I also bought a Samyang 14mm after reading the glowing reviews and got the worst lens I've ever owned.
Ya, I bought a 5D Mark II with results like that...
I don't want to lay out $1100 until a few reviewers that are careful and know what they are doing put up some reviews.
ditto, or I'll do my own tests
But I'm in no rush on this lens, appealing as it sounds optically, it is still a large and heavy bit of gear and I DO have a very good 100-400L for now.
Altho its resale value is likely to drop a fair bit over the next couple months if the new tammy is really good.
Hmmm, keep or sell? glass-gambling time...