I have been running the Bridgestone Duelers 255/55/18's for almost 2 years. Here in Oklahoma we get it all. Huge thunderstorms with torrential rains, flash floods all the way to the most recent ice storm that left us with 6 inches of solid ice on all the roads. I have to say that the Duelers have really held their own on all the slippery stuff. In rain they hold the road like a champ. In snow they are the best non studded tires I have ever driven on. Even when the conditions get slushy my X5 is still rock solid even in the turns and up the hills. As far as driving on 6 inches of solid ice I firmly believe that no non studded tire can hold well in those conditions. However I did drive 25 miles each way to work on that ice for a week with no incident. I drove VERY SLOWLY I have to admit and used mostly engine braking. I have to say the Duelers are much much better in wet and snow conditions compared to the stock Michelins they replaced. In fact they rock in snow and water. They inspire confidence and feel solid and safe. On dry roads I can't really tell the difference between the Bridgestones and the Michelins to be completely honest. The Bridgestones might be a little quieter, but they both hold the road pretty well overall when its dry and level. Hope this helps.

I just purchased a set of Sumitomo HTR sport @ tirerack 255/55/18 and had it installed at Costco for $40 this past Friday. Tires, mounting and balance cost was about $500.
It handled very well in the light snow/ice we just had and I'm very happy with the dry road performance.
Something to consider...

I am running the Bridgestone Dueler A/L Alenza tires in 255/55/18. They are a highway all season but perform quite well with good grip in both dry and wet. I haven't had the chance to test their performance in the snow. They are very very quiet and have an extremely high wear rating. I love em!

Well, I just placed my order with Tires.com for a set of Yoko Geolandar HTS G052 in P255/55R18. I was able to get the 4 tires with a tire warranty for LESS than TireRack's price for just 4 tires shipped.

Honestly, I've been a customer of TireRack for about 7 years and have purchased many sets of tires through them. I was truly torn between the two retailers, but the price difference was too large to ignore. It was TireRacks surveys that helped me make my decision though - Tires.com quality and quantity of surveys/responses are not nearly as good.

Once I get the tires mounted and balanced and a proper 4WA done, I'll let you all know how the tires are.

Regarding the nitrogen filling, just how do you think Costco is going to remove the air in the tires completely and fill them with nitrogen? Anyway, BMW issued a TSB on this issue and basically it's nonsense.

Well, originally I was not too impressed with my Toyo Proxes tires. I had them on for about 5k miles and they started howling like truck tires. The rears wore down to the belts within 20K miles. Come to find out I was in desperate need of an alignment. Purchased two new ones, took the two that were on the front and moved to the back because they were cupping (causing the howling noise), and had a good alignment done.

I must change my review - I love these tires. Great traction wet and dry. Recently got caught in a traffic backup and thought I would put them to the test. I crossed the grass median in the center of the highway that was muddy from it raining all day. The X did not slip one time. Even my wife was impressed!!

This certainly is interesting given that BMW issued an SIB on the Nitrogen debate back in April 2006 (see SIB 360406 attached):

BMW Accessories - 2010 X5 M SAV:Nitrogen Logo Valve Stem Caps