I used one for many years for my bird photography. Images from it can look a little soft, but they do sharpen up well afterwards so it isn't too much of an issue. The slowish shutter speeds can be, given the lens is F6.3. I used to find myself shooting at speeds of 1/60 to 1/250 of a second and subject movement was a major issue. I lost far more pictures because the bird moved and blurred in the frame then for any other reason. I had mine pinched from the car earlier this year, and was gutted. Having said that I used the insurance money to buy a Canon 500mm L F4.5 lens and I now find I get far more keepers.
The lens is very capable, and reasonably sharp. It focuses closer than my canon too. The 150-500mm IS may be a better option if you feel IS would help, but other than that I would highly recommend the Bigma. For the price I don't think there is anything out there to touch it (including the Tamron)