I've had XM in my van for 4-5 years now and you couldn't pry it out of my
cold dead hands.  There is nothing like being in the middle of death valley
and being able to pick anything you want to listen too... in that case it
was after full week on the road I happend to be mood for the comedy channel
for a couple hours...

As far as talk radio goes, I don't know what siruis has, but I listen to a
of NPR, BBC and Air America on XM.

I've very rarely had it cut out.  It'll only cut out if I'm on the north
side of a cliff (ie. southern horizon is a cliff face).  Not just
mountians... I mean up against a cliff.  It doesn't cut out when parked
under overpasses, although my GPS does.

My antenna is under the fiberglass of the luggage rack.   While I rarely use
the luggage rack, the reception got bad the one time I did put a ton of
stuff in the lugguge rack above the antenna.  That time it was cutting in
and out constantly.

-jon