Yes, the downloading of works from an Internet exchange market for private use in the personal sphere or in a private circle (Art. 19 para. 1(a) CopA) is permitted under Swiss copyright law as long as the works which are offered in the Internet exchange market have already been published. It would be illegal to download unpublished works as private use is only permitted for published works. From the perspective of the user, it is irrelevant whether they download works from a legal or an illegal source.
But it must be noted that the upload of works to an Internet exchange market is illegal because it is no longer covered by private use (Art. 19 CopA); however, an exception would be if the work is only used within the scope of private use in the personal sphere or in a private circle, i.e. among people who are closely connected to each other, such as relatives or friends. It is important to note that many file-sharing programs automatically download and upload parallel files.