Have had XM in all 3 of our Tesla vehicles (starting with our first in Jan 2013).
XM reception has been similar to previous non-Tesla vehicles - with issues when there isn't line of sight to the sky (heavy trees, under bridges, inside garages, …).
With continuous internet connectivity, the XM app could be improved considerably by using the internet connection.
This would significantly shorten the time to download the channel list (doing it on demand over the internet, rather than slowly building it from satellite downloads).
And it would provide access to many channels only available over the internet (such as the local feed for live sports events).