Currently the tamron is easily the best runner up choice to the canon.I think i can safely forget about the sigma 18-50/2.8 due to very lame performace at the long end plus too much CA.
... choice to ignore in body image stabilization ... i see now that Pentax have joined in with it's new K100.It has in body stabilization **AND** a huge number of great lenses to use on it ,both old and new
I'd agree about the Tamron, less about the Sigma tho. I have seen some lovely shots and maybe it is just at the extremes that you may get some issues. But just check out the price difference
The Tokina is the one that could really combine great build with superb performance for an honest price. Had three Toki's and all delivered. Exciting release ... more so than the Canon for me.
Agreed about the Pentax which I must check out. Looks very interesting from what you say. I am seriously very impressed with my 3 Pentax lenses.



