C2x, or check2xsf as it was first known, was originally written to convert CASTEP's .check files into .xsf files viewable with XCrysDen. It thus provided a means of viewing isosurfaces from CASTEP without needing to purchase the commercial, Windows-only, Materials Studio front end. An early enhancement, in 2009, added the ability to extract (co-linear) spin densities at a time when Materials Studio could not.
Since then this utility has grown considerably in functionality. Version one grew to read .check, .castep_bin, .chdiff, .cell and .pdb files, and write .cell, .cml, .pdb, .xyz, Gnuplot and .cube files. It could form supercells, generate k-point meshes, find primitive cells, and generate k-points. It could extract band energies and k-point positions from .check files. It could perform both Fourier and trilinear interpolation to plot densities on given lines through cells. It had some degree of support for writing .fdf, SHELX97 and .cif files, and reading .res files.
Version two of check2xsf, renamed c2x to reflect the growing functionality beyond simply converting .check files to .xsf files, merged cellsym and check2xsf. It represented a significant tidy-up of the code, with an elimination of most global variables. It also started to improve support for other codes, VASP and Onetep, being the first to be added, with Abinit soon following. Thus c2x can be considered to be a little like OpenBabel, but with an emphasis on different codes, and support for volumetric data. List of supported formats.
At the end of 2017 the source code to c2x version 2.05b was released publically under the GPL version 3. The detail of changes since then can be seen in the ChangeLog.
Although c2x is certainly used in the author's Research Group, it is also used much more widely nationally and internationally. In an attempt to track this usage, and thus justify development time, in 2018 a citable paper was published: C2x: a tool for visualisation and input preparation for Castep and other electronic structure codes, MJ Rutter, Computer Physics Communications, 225 174-179 (2018). It is too early to say how successful this approach will be...