I didn't notice any mpg change from studding, but the tires seemed to last longer. If you don't have a truck and can't run a good off-road traction tire, then I think that studs are by far the most cost effective and otherwise neutral to other considerations winter performance upgrade.
It's one if those FUD (fear, uncertainty, and doubt) things that once you do it you may feel silly for having worried about it - I did. Now it does add up front cost, but the peace of mind over studless tires on ice (which mostly suck) is huge.
Lots of people say the Michelin X-ice is very good, and I believe them, but on a heavy van on steeper grades, it's studs all the way. You may notice that nobody tests winter tires uphill. It's always flat surface acceleration. And what a tire does on a flat surface may, unfortunately, not correlate well to say hairpin switchbacks on a 7% grade. That's why off-road tires are so superior. They are designed to climb (and descend), and usually have far softer compounds than dedicated winter tires.
I will go ahead and post this here now that we are descending into tire wars. First up, 2007 Nissan Quest with traction control, FWD, Cooper Wintermaster S/T2 studded. Second up, 1995 Land Cruiser, 37" Interco Trxus MT (baby Swampers), AWD (viscous coupler), but basically RWD for the purposes of what you see here except when I hammer the throttle. At 1:15 I lock the center diff and hammer it and then slam on the brakes (ABS is off now). No reason to drive that way, but you can see the level of traction.
Now this is a difficult surface - powder falling at near freezing temps, packing, and then temps drop to low teens, so the ice is 'dry' with a little powder on top. But at a 3% grade, the studless Hankooks and Hakka Q's have been completely defeated in these conditions (I almost got t-boned in the Hakkas when they suddenly let go at an intersection at very low speeds). Studs give it a fighting chance, but that wouldn't make it up Berthoud in a shoulder season cluster when the pass gets really icy.
Somebody else will get stuck before I do in the van unless I am out alone in more Cruiser suitable conditions, but I sure would hate to be on a pass with exposure with my car doing what that Quest is doing on a neighborhood road.