There are too many variables involved to make blanket statements, which is why I just find offering up each person's experience and stating all conditions involved is best in allowing the original poster to make a decision. To say the lens is 'terrible' is as egregious as saying it's 'perfect'...neither is correct. As to how capable it may or may not be, or how fast or slow, so many other variables have to be taken into consideration.
First, DSLR bodies range from slow autofocus to blindingly fast - so to say a lens is too slow doesn't take into account that different camera bodies have quicker focus motors than others. What might have been uselessly slow to focus on one camera might actually be significantly faster on another, and even capable of keeping up with tracking movement across the focus grid.
Light is most certainly a factor. In poor light, the Tamron is certainly not the lens to lust for - but some cameras can focus much better in poor light, and shoot cleanly at much higher ISOs than others, so what is 'usable' in a given light also depends on one's camera body, not just the lens. I didn't see the original post ask if the Tamron was better than the Sigma, or any other lens - it seemed to just be asking if bird-in-flight shots were possible with the Tamron. One must consider those variables, and then take the user comments offered here into consideration, to help decide if your own camera, and the conditions under which you will likely be shooting, as well as the subjects you intend to shoot in flight, will be handled acceptably by the Tamron, or if you need to consider a much faster, much more expensive lens to arrive at your goals.
As for my experience, I disclosed all the reasons the Tamron works very well for me with birds-in-flight...in order to not deceive the original poster. I live in Florida - good light is in abundance. A majority of the birds I like to capture in flight are typically larger than a golf ball - some quite large indeed. My camera body has a very fast focus motor that has no problems spinning the big Tammy lens and keeping focus on moving targets - even those moving towards me. My camera body has stabilization, so in effect the Tamron lens is stabilized for me, which equalizes it well against the Sigma, and allowing me to shoot handheld and with moving or skittish targets.
How much skill a given photographer has is certainly something to consider - amateur vs professional photographers can get wildly different results - and there are many levels of skill within the definition of 'amateur'. And how one intends to use the photographs also matters - the rather raw and harsh criticism above of some of the photos might be apropos coming from a professional photographer who must meet publication standards with their shots, talking down to another aspirational professional photographer trying to get published. But if the person is just looking for photographs to document their sightings, make small prints, and share with friends or enthusiast boards, then the criticism above to me was off-base, and unkind, and certainly undeserved. And the recommendations of this lens should take those factors into consideration as well. Likely noone referencing the 200-500 lens in good light is trying to recommend this lens as the best birding lens on the market and the choice to make for becoming a resident bird-in-flight photographer for National Geographic...just that the original poster asked for anyone who has used this lens to offer up their experience and opinion. Those that found it unacceptable for bird-in-flight shots have offered their experiences, and those who found it acceptable for their use offered theirs. Neither side needs to start throwing around blanket statements as facts from upon high that the lens is excellent or terrible. Let the original poster decide whether the lens meets his needs or not...that's all.