I cheated and downloaded the Windows Phone 8.1 Developer Preview instead of waiting for my carrier to roll the update out. The only casualty is a broken Facebook app (which works fine after reinstalling). Cortana works perfectly smooth despite it running on a lowly Lumia 521 (the cheapest Nokia WP8 phone). However it does not entirely speak in Jen Taylor's voice yet. But according to Microsoft, the default voice will gradually be phased out for Jen Taylor's.