Not owning one but would if I could find a used one (hard to find), people don't sell them., I'd have one in a minute. I'd say with proper care, they will last a lifetime. Not all that fond of the roof lift system as it intrudes in the living space but other than that, I see no negatives (other than new pricing).
I run a pop up now but it's a Palomino SS1500 which is all optioned out. I had a Lance HS and I immediately noticed a huge difference in driveability, especially in cross winds or when passing a truck on the highway. There is no buffeting, no sway, nothing. In fact, don'\t even know it's in the bed unless I look in the mirrors and we primarily camp off road, unimproved and it's super easy to off road one.
Far as mileage goes, I experienced maybe a 1/2 mpg drop overall. The low transport height makes for a much more efficient profile.
Downside with any pop up (and the Alaskan as well) will be storage room. Pop up's by nature have limited storage capability. Glad I have a crew cab, my backseat doubles as storage though only half because out dog gets the other half.
Would I ever go back to a hardside? probably not. An added benefit of the pop up is, I keep mine in my garage when I'm not using it. Out of the weather and will last decades. No hardside will fit in a conventional garage.