Some good advice to be found in this thread.
My primary enjoyment is wildlife photography. I like to go birding, and when I decided it was time to expand my photography to that area, I was shocked at the prices of a relatively fast prime lens in the 500mm range.
I eventually settled with the Bigma as it was at the right price for me. That and you wouldn't imagine how useful having 50-500mm on tap can be when your trying to photography unwilling subjects.
However, it is a relatively slow lens, and I have learned to work within its constraints. It is all about compromises if you’re not a millionaire. With these slow lenses, I found I had to up my ISO to 800 to get my shutter speed to around 1/1000th to stop the action on the following duck photo taken with the Tamron lens mentioned below. This is the compromise with slow lenses that you will have to make sometimes. You could always use a flash, but then, light reach becomes your enemy for wildlife photos. You might have to get a better beamer or some like attachment to get that extra reach.
Now, my father bought a Tamron 70-300mm lens to fit the K10D. If I knew what I now know today, I would have talked him out of it. Yes, the Tamron does have slightly better IQ, but at the cost of terrible purple fringing from chromatic aberration. Yes, it is relatively easy to remove in post processing, but honestly, is it worth the time to remove it for all the photos you want to keep? Today, I would honestly go with the Sigma 70-300 APO in a heart beat. At first, I was sort of happy with it, but not after the latest duck picks.
Here is an example using the Tamron 70-300 on the K10D. I was terribly disappointed in all that purple highlighting in the water drops. I didn't bother removing it either.