I agree with Dave -the doughnut effect can look quite attractive - especially on water when it's so obvious it doesn't look like some accidental aberration and it's quite easy to post process out any effect you don't want.
The main advantage is the mirror lens is it's more affordable - although it does sometimes tend to get a viscious knocking by some gear freaks who will sell their soul for better/bigger/swankier stuff.
There are still a few wildie pros out there who use mirror lenses - don't knock them too much. Like a lot of things they have their advantages and disadvantages.
They're a lot shorter and easier to handle but you do tend to get stuck with f8 which is a bit slow and more or less rules out use on a dull day.
Re lenses - you can have an 800mm lens and it still won't beat getting close. Although it is useful if you're big and strong enough to heft it up quickly. that rules me out then...
Successful Wildlife Photography rules -
1 get close
2 get close
3 get close...................
Acherontia