I have the Sigma 600 f/8, it is not brilliant, but it isnít a slouch either. I bought mine second hand about 22-25 years ago and used it extensively to shoot kangaroos.
It was the only way I could get a 600 mm lens, I paid $180.00 AUD for it.
I made many a memorable slide with this lens, and squillions of colour negatives, from which I made some quite good prints.
You donít mention what camera system you are using, but if you are in Nikon land then a reasonably cheap way to get a relatively quality lens kit is the Nikkor 300 Ais f/4.5 with internal focusing, couple this with a Nikkor 1.4 converter of the correct type and you have a reasonably compact 450mm focal length lens with virtually no degradation of image quality. This is the route I would go today.