I have recently discovered this method. I always had an interest of vintage sound reproductions. I was fiddling with my records and makeshift horns and was plesently surprised that it can actually reproduce sounds!
The sound quality is not as superior as you can hear via electronic speakers, but hey, its only made out of paper! :P
The best records to play with are wider groove records- microgroove records doesnt tend to respont too well as the "needle" on the horn is wider than a microgroove. You may also want to try it on original 78's (they have even wider grooves) as those were originally played in a very similar way. I dont own a 78 so I never tried it.
It works best with stronger paper- about 100-120gsm(not card though). Also, you shouldnt press the needle too hard. The needle (and horn) will only last for several minutes before the needle gets blunt- this can only be used for several minute's fun. To be honest, needles in gramophones only lasts for one play anyways!
You know when the needle gets blunt as the sound quality/volume has reduced and sometimes the needle will fray out and ride on 2 grooves (listen carefully to the very end of the song!)
Next time you have a power cut, you may want to use this method (and turn the turntable by hand!) xD