Sounds like not a good experience, but you don't have to only use the 65mm at 5x since its variable (1x to 5x), which provides greater flexibility as compared to other options discussed in this thread. Also, the 1DS3's resolution, monitor and live view are inferior to the X1D, so maybe not so applicable.
I used to own the 65mm, but sold it a couple of years ago on leaving the EF ecosystem (and only recently acquired an X1D2), so I can't comment on its use on the X1D. While the 65mm is a MF lens (so even a dumb adapter would work), another unknown is if the TECHART adapter allows camera control of the aperture. It should; but if not, the 65mm would only shoot wide open.
And speaking of other discussed options, the Zeiss Luminar 16mm (10x-40x), 25mm (6x-20x), and 40mm (4x-10x), quoted in an earlier post, have working distances (and lighting issues) even more minuscule (and are probably more expensive and cumbersome to use) than the Canon 65mm. Use of the diffuser I referenced could help with the lighting, so maybe the 65mm is a viable option for some > 1:1 users.