I have been a avid C4 fan since I started climbing trad, and have traditionally (pun intended) doubled up in the C4's - sometimes over other cams of lighter weight.
When these came out, I was at first slightly concerned about durability. Then I started thinking about them in terms of application and durability and where ultimately they fit into my rack and came up with a few great features.
1) They are not, and never will be, the workhorse of my rack. That job falls to the C4's and Totem cams that have always held that coveted spot. The Ultralights are my "doubles" and "triples" in some cases. They are not the cam I reach for when I am cruxing - while they would do just fine (i know because I actually fell on a .75 - yes, I fall a lot when trying above my pay grade) - they often see less action. I think about them as a supplemental piece - because usually I am not falling, and in the chance that I do fall, what is the chance its on an UL and if I did they hold a fall just great. The thinking here is to increase the longevity of the UL's.
2) Weight savings. I am surprised at how much lighter these are to their C4 counterparts. The #4 is so much lighter than the OG - i'm able to leave it on my rack permanently, where before I would leave it in the truck or at home. I can make more open route decisions up at the crag with a more full rack. The ability to double up in these is great.
3) Where they shine is in the alpine, or on long approaches. First is the mentality of the alpine - "The leader doesn't fall" - that being said, IT HAPPENS. but for the weight savings and the off chance that you do fall, these are trucker.
The biggest questions it seems everyone has on these is LONGEVITY OF DURABILITY. If these are used correctly, i.e. - you really shouldn't have to fall on them - they will seemingly last a long while. But if you do find yourself, like me, slipping and falling one of these - have piece of mind that they held me like a champ.
The C4 got a hair cut. and they are looking stylishhh.