It´s great to use vintage lenses. Helios, Takumar, Mir, Nokton, Canon LTM, Pancolar, Jupiter? They all have their own signature and are fun to work with. With the right adapters - cheap and easy available on Ebay - you can adapt nearly any old lens to any modern camera.
Some criteria for good vintage lenses:
* Price : Helios lenses - the Russian copies of the German Carl Zeiss Biotars - are widely available and affordable. The Canon LTM 50mm f1.8 is another great lens, sharp but with its own bokeh, widely available. Excellent lenses made by the Japanese manufacturer Tomioka like the Auto Rikenon 55mm f1.4 are somewhat more expensive but still don't cost an arm and a leg.
* Effects and bokeh. Some lenses like the Helios have a swirley bokeh. The roundness of backlights is heavily influenced by the shape and number of aperture blades. Some lenses produce a bubble bokeh.
* Size. Leica Thread Mount (LTM) lenses and lenses with an Leica M-mount are usually lighter, smaller and thus easier to carry around. Jupiter lenses are the Russian copies of the German Sonnar type. In the fifties and sixties Nikon, Minolta (M-Rokkor) and Canon (LTM) made excellent lenses for their Leica like rangefinder cameras.
* Clean. Some dust particles between lens parts shouldn't be an issue, Fungus is! Please avoid those lenses at any cost. The aperture blades should be clean and functioning correctly.
* Easy to use. The focus ring must be easy to turn. Some lenses have to be lubricated before use (not allways an easy job). Some lenses have a large focus throw and it's a hell a job to adjust the lense from nearby to infinity.
* Radioactive. Some lenses like my Super Takumar 50mm F1.4 are radioactive. On Youtube you can see which lenses are radioactive and to what degree.
* Coating. My Yashinon 50mm DS 1.7 is a single coated lens. It's prown to flare but great for black and white photography (less contrast, more greytones).
* Sharpness vs dreamy looks. Modern lenses are sharp but often give sterile results. Old lenses are loved for their dreamy looks while used wide open.
On Flickr there are thousands of pictures made with specific vintage lenses. Check it out!