Recently, I had a number of day and night flights using XM's nexrad in the TBM in Texas and I found it very accurate to location.  Reference this one picture taken at FL270 just after passing a group of cells.  I find the portable G696 of great value as it can be removed and put into any aircraft.  On the last trip from Florida to Calgary in the Legacy (almost all VFR), the G696/XM proved indispensable to find good VFR stops down the road and upcoming weather.