I have the Xi3 on my '09 Outback. I am very impressed with them. I live in northern Minnesota, so I've already had the opportunity to use them in lots of ice, snow, and slush, and I have to say I'm very pleased.
Dry performance is quite good as well. To be fair, these are the first snow tires I've ever purchased, so I can't compare to other dedicated snow tires, but I honestly didn't feel a huge decrease in dry performance vs the stock Potenzas.
I also haven't noticed any hum that TireRack talks about. If anything, they are quieter than the stock tires I had.
Efficiency is very good, in my opinion. I've experienced very little, if any, drop in mileage vs. the all-seasons. If there has been any drop, it's been on the order of 1 to 2 mpg, but it's very difficult to tell if that is a result of the tires or the colder weather we've been having, which always seems to come with a corresponding drop in mileage regardless. During Thanksgiving, I drove it about 400 miles round-trip in a day on all dry roads, and I calculated a mileage of 26.6, which is very good for that generation Outback with the automatic.