I also have the Kimplex Nomad Seat/Storage box. Mounted with the Lock n Ride Studs from ATV Superstore.
Wife is very happy with it, Grandkids love it, even my SIL rode with me and said it was very comfortable. The storage Box holds a BUNCH of stuff.
As to being "legal" or not, well if you ride on Private Ground it does not matter. On State Trails they have their own rules and you are supposed to follow them. However having said that I know of lots of guys that use them and never have an issue-- but IF the OFFICER that happens to be there checking wants to give you grief they can.
Learning all over again.