Hi Peter
A lot depends on how often you will be using this glass and for how long at a time. Compromises are inevitable if weight and/or cost are a factor in your decision.
For me, the weight of lugging a heavy super tele-zoom was a consideration consistent with adequate image quality. I tried the Sigma Bigma and weighed the Sony 70-400 and settled on the Tamron 200-500.
It needs to be stopped down a couple of stops but is more than adequate at F8 better at F9 or F10. In late autumn trudging around England's New Forest, light is not very good but I was still able to get some acceptable shots of wild deer, handheld. However I did have to eventually use a higher ISO of 800 than I usually use, to get decent results hand held. This lens can be hand held down to 150th with the A900, the steadyshot system still works very well indeed. It is sharp at the full zoom of 500.
There are several independant user reviews of this and other lenses over at the Dyxxum web site.
Bokeh is nice, without ugly specular highlights. It is not a top notch lens but for 499 GBP (used) it is exceptional value, and weighs not much more than the 70-300G. With the hood attached it is a very long lens at full zoom.
This lens is in current production. Tamron's should not be discounted as they do sometimes beat Sigma's and manufacturer's lenses in independent lab tests. Like every lens manufacturer (including Leica) they do make some "dogs", but this lens is not one of them IMHO.
