The best place to find out about this is in such documents as the `Linux Info Sheet', `Linux Meta FAQ' and `Linux FAQ' (see Linux Documentation). Major reasons for a visually impaired person to use Linux would include it's inbuilt networking which gives full access to the Internet. More generally, users are attracted by the full development environment included. Also, unlike most other modern GUI environments, the graphical front end to Linux (X Windows) is clearly separated from the underlying environment and there is a complete set of modern programs such as World Wide Web browsers and fax software which work directly in the non graphical environment. This opens up the way to provide alternative access paths to the systems functionality; Emacspeak is a good example.
For other users, the comparison is probably less favourable and less clear. People with very specific and complex needs will find that the full development system included allows properly customised solutions. However, much of the software which exists on other systems is only just beginning to become available. More development is being done however in almost all directions.
There is almost nothing commercial available specifically for Linux. There is a noticeable amount of free software which would be helpful in adaptation, for example, a free speech synthesiser and some free voice control software. There are also a number of free packages which provide good support for certain Braille terminals, for example.
Linux has the vast advantage over Windows that most of it's software is command line oriented. This is now changing and almost everything is now available with a graphical front end. However, because it is in origin a programmers operating system, line oriented programs are still being written covering almost all new areas of interest. For the physically disabled, this means that it is easy to build custom programs to suit their needs. For the visually impaired, this should make use with a speech synthesiser or Braille terminal easy and useful for the foreseeable future.
Linux's multiple virtual consoles system make it practical to use as a multi-tasking operating system by a visually impaired person working directly through Braille.
The windowing system used by Linux (X11) comes with many programming tools, and should be adaptable. However, in practice, the adaptive programs available up till now have been more primitive than those on the Macintosh or Windows. They are, however, completely free (as opposed to hundreds of pounds) and the quality is definitely improving.
In principle it should be possible to put together a complete, usable Linux system for a visually impaired person for about $500 (cheap & nasty PC + sound card). This compares with many thousands of dollars for other operating systems (screen reader software/ speech synthesiser hardware). I have yet to see this. I doubt it would work in practice because the software speech synthesisers available for Linux aren't yet sufficiently good. For a physically disabled person, the limitation will still be the expense of input hardware.