CONS
There are of course disadvantages to using those old lenses:
  • You have to focus them manually.
  • You have to set the lens aperture manually (though many dslr's will work in aperure priority mode and set a suitable shutter speed for you).
  • They are often heavy (because they are made of metal - not flimsy plastic).
  • You will have to think about things like f stops and depth of field, etc..
  • They may have dust, haze or fungus problems, (don't buy or use them like this).
PROS
The pros, I think, outweigh the cons by a large margin:
  • Artistically you will feel more in charge of your photography.
  • Slowing the photo taking process down will make you notice more about your subject.
  • Many of the older prime lenses have less coatings on their elements, the colours in your photos will look truer and crisper.
  • Taking the time to focus manually will allow YOU to select the focussing point in each shot.
  • You can own and use a whole collection of lenses rather than the latest single offering from your camera manufacturer or even a third party maker, at a fraction of the cost.
  • Constucted of metal these lenses are much more robust than their modern plastic counterparts, and the prime lenses in particular are often 1 or 2 stops faster.
I have two collections of these 'old' lenses, one with the Pentax K mount and another with the even older m42 screw mount:-

www.ipernity.com/doc/welshgog/33661973/in/album/677231
www.ipernity.com/doc/welshgog/33689983/in/album/677231

though both of these collections have grown slightly larger since the photos were taken, and also an album of photos taken using them:-

www.ipernity.com/doc/welshgog/album/677231

I hope I've given you a little food for thought, and perhaps the enthusiasm to try a few of these great lenses for yourself.