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1) First of all. Forget the "bad reviews"... forget the reviews in general. If you like the way your lens works, enjoy taking photos. You need to test your lens and know the specific flaws you detect. Ok then you can read a review to understand what you already see.

2) In my humble opinion. Manual focus in a digital camera without a focusing screen is a pain. Any improvement you have in sharpness you can have, you will loose it focusing the wrong distances. Specially in wide apertures. Not even with live view.

3) Depending on how old are we talking about. I have some old interesting prime lenses. Old enough to have some chromathic aberrations on all of them, things that I did not noticed when using film back in the days.

4) You will use a manual lens probably in a studio with controlled situations, a tripod, a still life that is not moving. If not, use an auto focus lens. There is no hurry. But if the lens is really cheap, go and buy it and have fun with it.