Military.
Well, if possible, various militaries will make use of it. Probably first in planning of operations. The current version is not suitable for deployment in a field.
But the first adopters in the field will likely be those which need assisted exploration, so either medical or resource extraction (e.g. mining, drilling, sea-bed exploration, etc.)
If a MIL-SPEC version were made it will cost much more than $3.5k. And security for the software will be a big ticket item (e.g., a unique version of the operating system.)
Even if there is no field-worthy version, in operation centers a VR headset can find uses is planning. Ideally a field-worthy version could be deployed in a platoon for maneuvering in say a cityscape. Air traffic control is another obvious application (for civilian as well as military.)
Lots of educational uses in science and engineering.
And I do think that civil/construction/structural engineering as a field may soon make VR headsets standard in day to day business. And of course architects will love such.
Eventually I can envision real estate sales using these things, to do walkthroughs.
But we all know what you young guys (and most people on these forums are male) will use the Vision headset for: porn.