It's not a bad lens, but the OS model is better, and it works better with teleconverters.
If you don't use teleconverters and you know that you don't need OS, you may be better off with the non-OS for half the price but then of course you have to be comfortable with living with 300mm (which is actually less than 300 with near subjects.)
However, you mentioned wildlife shooting and that you had 420mm before, so the old non-OS Sigma won't cut it without teleconverters.
Check out my nature shots portfolio (down in my signature) to see hundreds of shots taken with the OS Sigma and teleconverters.
Don't like at gabebalazs profile, if you do you are going to buy it.