“It works”, applied to any inanimate object is probably the more common expression for the basic meaning, that the device is able to function (not broken).
“It is working” is probably meant to imply something in addition to the basic meaning. If the speaker stresses the “is”, that would make it certain that they are implying something more. For example, the speaker might mean:
- It is being used right now, the photocopier is currently making a copy.
- The speaker is surprised that it is functional.
- It is functional now, but it wasn’t before.
- It is functional now, but it might break again.
Addition of an exclamation point or expletive would confirm surprise. A speaker might stress the “ing”, to clarify that the object is not just able to work, but is functioning at the current moment.
Of course, if the subject is a person rather than a device or tool then the meaning is completely different, “he works” meaning that he labors, is employed, has a job, and “he is working” meaning that he is doing one of those things at this particular time.