K1, K3, and DXG going strong here. (shrugs) Never had issues with rebooting or the like. I probably shouldn't say the Kindle 1 is going strong, I have not turned it on in a year but the last time I did, it worked fine.
The reality is that most of these are being made and replaced so quickly that I doubt you are going to find too many people using the same device in five years.
The PDF capability on all of them is poor, PDF just doesn't render well on a 6 inch screen. Collections/Folders/and the like are all going to be slightly problematic and very much a matter of personal taste. I am fine with collections, it was easy enough to set up in a way that made sense to me.