I have finally taken delivery of the Tamron 150-600/5-6.3 Di VC USD zoom lens after a three month wait. Prior reviews have been mostly positive calling it a bargain at the price. I hope to start testing later today. The focus speed and the vibration reduction seem to perform above expectations thus far. While it is not a lens I will use extensively the extra reach will come in handy on some shoots. I have read performance is great up to 500mm and then starts to taper off a bit.
UPDATE
Initial impressions are very favourable though this is definitely a heavy lens to carry around. The VR function seems to work extremely well and focus is quicker than expected. Sharpness is excellent from 150mm to 400 and 500mm is very good. 600mm is definitely usable in pinch though I would rather stay at 500mm or lower!