I recently bought an inexpensive adapter that allows me to put my old Pentax Super Takumar 50mm f1.4 lens on my Canon XSi Digital SLR. Why would I do that? Well, it is a very good lens that has not gotten any use in probably 20 years. It is fast allowing photography in low light. And because it is so fast (has a very large maximum aperture opening) it produces really nice shallow depth of field as is demonstrated here. The blossom in the foreground is tack sharp, and the backrough is nicely out of focus making the flower really "pop".
Using this old lens has many trade-offs. It is a prime lens, meaning you cannot zoom. It is manually focused, which can be tricky especially when using it with the large apertures. And the apature must be manually set, so if you want to shoot with a smaller apature (for more depth of field and less critical focusing) the image in the viewfinder gets very dark.
But it is fun to bring some life back to old equipment and take time to create images that would not be possible with the modern lenes that I own.