I installed a swingcase on the driver side a while ago, maybe ~4 months ago, and a passenger side one about 1 month ago. I really love them. But I guess to help you out as much as possible I'll give you the long story, lol.
First thing to say is that undercover is a great company, and really took care of me with the issues I had with their product. I wanted the swingcase(s) because I haul my buell in the bed of the truck, and didn't want a conventional box in the way. I bought the driver side through autoanything(also great to deal with) and after install, the box would barely latch because it rubbed on the tie down hook. I contacted them and got a response after a few days which I thought was reasonable and they said they had never had anyone report this issue. The offered a refund, but I told them I had already drilled 7 holes in my bed(and linex) and I was going to make shims and install it. At that point they offered to give me the opposite side for free, for my troubles. I almost said no because it took me a few hours to make nice shims(I have limited metal working tools) but I took them up on it, and now have both installed. I had to shim them out about 3/8" to get clearance. Other than that the only thing that is at all bothersome is that I had to do some touch up painting on them from where the screws scratched the paint and started to rust the bracket. To be expected with steel though.
In the end, it was a little bit of a PITA due to having to make shims, then having to buy longer screws, and having to do some painting, but I am very happy with the finished result. And the best part is they will stand behind their product.
One good thing about the shims is that since it pushes the box further out from the side you can open the lid further(nearly all the way to 90 degrees) before it hits the bed rail.
Also note, you lose access to the tie downs with these boxes.
As for durability, it is durable ABS plastic, and so far has held up fine(for these months). My truck is outside all day everyday, and gets washed maybe 1-2 times a year(the boxes have yet to be washed). The only thing that happened with mine is I scratched up the "swingcase" gel sticker on the box because I packed the truck full of fire wood and I guess it rubbed on that gel sticker and marred it up. I'll probably just take them off eventually anyway, I don't like stickers.
One other thing, I used the self tapping screws(provided) for the first box, but it makes the holes fairly large and you can't get a really good bite. On the second box I predrilled the holes smaller then used the new SS screws I bought from ACE(that were longer, for the shims) and I got a much better bite. I'd suggest doing this.
Hope this helps, if you have any other questions or need any help let me know, or pictures or whatever.