FYI, diamond files have an abrasive grit on them, whereas files for fine work (single cut) have rows of smooth, sharp edges. This means that single-cut files leave a much cleaner surface AND remove more material in most applications. Diamond files are only advantageous for materials that normal files won't cut, like hardened blades, and the surface they leave needs further finishing in most cases. I have a set of diamond needle files but reserve them for hardened blades only, and this is tedious work, so I avoid filworking hardened blades if possible.
I do most of my filework with three files: a half-round SEARS needle file, a cheap "cat's eye" (flattened oval) needle file, and an 1/8" chainsaw file. I recommend getting a full set of decent needle files as you may want each kind for various things, plus 3/16" and 1/4" chainsaw files (chainsaw files amaze me - they cut incredibly fast and leave a very fine finish). The trouble with most needle files is that they have "safe" or non-cutting edges, even triangular files. The cat's-eye pattern is the handiest I have ever seen, because it cuts a very narrow notch with the edge. Unfortunately, I found it in a discount store and have had trouble finding another. I'm also playing with a 60-degree thread-restoring file for cutting notches.
-Drew
