Very good, but it's better

It is no longer faced with the question 500mm prime or not, but the question this excellent lens or at least to acquire the very cheap 50mm F1.8, here a few hints. 1.8 to 1.4 are 2/3 stop. So roughly speaking not quite twice as much light, or a doubling of the ISO level. Need this photosensitivity?

At aperture 1.4 the depth of field level is only a few centimeters. For portraits you have to make sharp to her face, anyway it a little close.

This lens, although not very sharp coming, ran to the power of 1.8. So if you do not need this 2/3 stop, you probably should buy the 50mm F1.8. It is also much cheaper, what one also should not be despised.

Both also focus about the same speed.