I would regard myself as a "Big T" veteran - now almost a year & a half of using the lens - with about 30,000 shots ( on my faithfull D70 ) . I shoot almost exclusively at the long 500 range and i cannot remember when the lens has left my camera .....
I've written my opinion a few times in different threads- so i'll add from my experience:
I find it to be a great lens for the money ( just like Rezmole said ), with a tremendous advantage of weight : It is easily hand - held .
I have had the Sigma 170-500 and i find the Tamron far better in all parameters.
Like all other long lenses - it loves good lighting conditions but there's no problem shooting in low light . ( the lens tends to " hunt " in low light ) .
As for the Halo seen in the above mentioned photographs : as far as i can assume - the halo is a result of quite a bit post-processing of "Shadow\ Highlight " + over sharpening . You do not get such halos from the lens . ( i've never encountered this ) .Futhermore - there's almost no CA .
The lens is VERY sharp and to my personall opinion - it performs best at
F\8 , - with very good results at F\7.1 ---> F\9,10
It is not a fast lens but with practice - you can manage very accurate and split second shots like this :
http://www.birdforum.net/pp_gallery/showphoto.php/photo/89336/sort/1/cat/500/page/3
MY GALLERY shows the results.
I hope i've been of some help .
Here's a result of shooting in harsh lighting + very good sunlight :
The spurs -no sun at all ,and the kingfisher - shot at dusk
The White throated: shot with very good sunlight.