Aside from the benefits to the passenger using their device (i.e. communication capability) the effects of cellular or portable electronic usage in flight are almost all negative. The most reported event is the electronic beeping sound that is heard through a speaker when a cell phone is held close. This sound will transmit into headsets and could make it difficult to hear your crew, or ATC instruction. Other reported issues caused by cell phones that are less common include navigational system malfunction, erroneous instrument readings, and unexpected nav failures.
Hope this helps,
James
