Currently, if you want 600mm there are 3 options
1) Tamron 150-600
2) 300 f/2.8 + 2xTC
3) Minolta 600 f/4
The second is too expensive for most, and the 2xTC will degrade IQ.
The Third means waiting for a reasonably rate s/h lens to come available, and then hoping the price and condition are good (but will be the best option IQ wise)
This leaves the Tamron as the only practical option, and as you say, reviews are fairly positive.
Sigma will be brining out their 150-600 Contemporary in Sony A-Mount eventually, but I would guess it will be 6 months + before we see it.
The other possibility is the Sony 70-400G and cropping - which gives better results at 400mm and below, and similar results above 400mm (IE the cropped results from a 70-400G are comparable to the Tamron), but costs significantly more.
Personally I'm leaning towards getting the Tamron (soon), but if a good 70-400G came up s/h I might be swayed!