I am truly in love with this lens. Mine may be a bit older than the one pictured here. It does not look exactly the same, but the specs are identical. I was looking at the 24-70 2.8 L and happened across this one used at my local shop. I had a 30 day money back so I tried it. I have never looked back.
Maybe build quality varied with this lens but mine is solid, built like a tank and as heavy as one. This lens is almost to sharp in some cases if that is possible. At f/2.8 I have had mixed results but anything under that has been fantastic. I have been into photography since the early 80's and this is my first non-canon lens. As you can read into my review, I have been impressed.
My only real qualms with the 28-105 have been the auto focus. It is a bit slow, hunts in low light, has trouble with dark objects in full light sometimes, and can be a little noisy. It sounds gear driven...
Before buying one of these, it looks as if you should try it out first. I believe Tamron's MSRP is about $1200 when this lens was new.