You don't have to replace the mount and can get away with a simple ring adapter if you adapt the lens to a mount with a shallower registration with mirrorless cameras, such as Sony NEX, Fuji X, Canon EOS M, or micro four-thirds.
For mirror clearance with a Canon prosumer full frame, you can also, if you don't care about resale of the camera, and want to use the full focus range of the lens, "shave" the mirror (i.e., grind it down to make it shorter so it doesn't come into contact with the rear of the lens). Or, of course, only consider using these kits with a 1-series or crop body instead.
But that's not the end to the inconveniences. As you've noted, you will have to focus manually. That can be tough with bad eyesight, or with a dSLR, since the viewfinders in them no longer contain the manual focus aids we used to use in film SLRs. Liveview with an LCD might help with zoom and focus peaking capabilities, but then you're composing at arms' length to see what's on the LCD. And you have no electronic communication between the camaera body and the lens.
This means you won't have any lens EXIF information (focal length, aperture setting used, etc.) You can't shoot in any modes other than full manual and aperture priority, because the camera can't tell the lens to change the aperture setting. You have to have a camera with stop-down metering capabilities, or you won't have accurate metering (most digital cameras are designed to meter while the lens is wide open, and then to compensate the metering reading for the aperture setting--without actually stopping down the lens. This way you have the most light possible to compose and focus by. Without knowing what the lens's aperture is, the camera has to then switch to metering solely by measuring the amount of light coming in, and your view can get darker as you stop the lens down and less light comes in.
Whatever monetary savings you may have had comes back to you in inconvenience. Unless you have a love of vintage glass, a need for the larger manual focus throw, or just a luddite like spirit backed with sheer cussedness to do a thing just because you can, this may not be the path for you.