another thread going right now:
http://www.xoutpost.com/bmw-sav-foru...tml#post968595

Don't blame the car for the tires' lackluster performance. Some all-seasons are better than others, and apparently your gf's Falkens are better than your Yoko's.
And like JCL says, you have a few things going against you. Your X5 is almost twice as heavy as her Sentra, which means twice the inertia you have to stop, even if you had the same tires, that alone would mean it should take quite a bit longer to stop (in low traction). Wider tires are the opposite of what you want for snow. Her Sentra probably has 195's? And your AWD means you have twice as many (+1 more) rotating masses adding rotational inertia also making it harder to stop.
Unlike JCL though, AWD should always be better at accelerating in snow. My horribly slow 325iX is a beast in the snow and must gain 50HP when its below freezing. My only complaint about the snow tires is that it requires alot more effort to do any good drifting in the snow now.
**misread JCL's post - yes, any car with snow tires will be better than any car without snow tires**
Tire rack has many good video reviews/comparison of snow tires vs all seasons. And pay attention to the comparison of studless vs studded. Studded tires are an unnecessary evil, unless you are ice racing.

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Olympic Peninsula, WA, USA
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2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i