I've been using this Tammy for one month now, mainly with my D4s : this is a fantastic combo for most of my usual shots. Bokeh, colour rendition et sharpness are amazing from such a "cheap" high range zoom-lens.
For me, the biggest weakness of this Tammy is its VC (stabilisation system) : not as good as my 200-400/4 or 600 VR. So, most of time I turn the VR off and use high shutter speed (this is not an issue with the D4s !).
As a bird photographer, the Tamron 150-600mm is an excellent walk-around lens for me.
A few more shots from yesterday, all with D4s and handheld :