The 1.5.x series is still experimental, but it is where most of the Due support lives. We generally advise people to stick with the latest stable version (currently 1.0.5), but that isn't always an option for people using the Due. Honestly, Due support is kind of spotty right now, and will probably remain so until the 1.5.x branch becomes stable.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.