As a few others here have shared, I've also tried the current version of the Tamron 28-300 zoom and quickly got rid of it due to poor image quality. I convinced myself that a used one for a good price would make the IQ acceptable, no go.
Personally I like the idea of a light all-in-one travel (vacation) zoom. The 28-300L has impressive IQ for it's range, but the size and weight just makes it generally prohibitive for me to use it as a travel lens.
Having been burned once with this Tamron, I'll wait until the dust settles and real world reviews are in before I get to worked up.