I'm currently working on a project that is using Ignition, from Inductive Automation. Since Ingition is Java based, OPC-UA is essentially the only option. From an HMI developer perspective I can't tell any difference. I suppose that's a good thing.
I've actually never liked OPC's reliance on the COM protocol. It's outdated, undocumented, not standardized, and not truly multi-platform. So I like the idea of UA. However, I have to admit that OPC has worked so well for so long and is so ubiquitous that it's a little never-wracking to try something so radically different. I wouldn't really want to be the one to switch, say, an entire plant over during a down weekend.
