I bought the Tamron 70-300 LD Di Macro.
I has plenty of reported faults but I think plenty going for it when used carefully.
The AF can be slow and you do get fringing in high contrast areas but nothing that stops me using the lens.
Here are a couply of vastly different shots taken with it
Sigma offer a pair of lenses that are roughly comparable in optical quality and price but in the end and after much head scratching I plumped for the Tamron and have not regretted it.
It and the Sigmas are a good cheap Telephoto introduction for those who can't afford the much much more expensive Pro lenses.
My tammy cost £69 - an absolute bargain!
Regards MZ