I've been surfing on sit on tops for 4 years and bought the Malibu Vision 3.4 kayak to use on green rolling surf/the sort of lazy, low power wave we often get along the coast of South Cornwall post gales and storms.
What is good: The Vision tracks brilliantly in lazy surf. It is fast and it is possible to keep it on the 'sweet spot' as long as the wave holds. It also quite good on flat water and tracks straight without any fuss.
What is bad: The fittings were are nightmare and simply not up to practical use in surf (for which it was advertised). The fittings securing the thigh straps were held with light pop-rivets and plastic U-clips. The clips quickly broke when rolling the kayak and then rivets starting pulling out! Not safe and a real risk to the paddler. The shop replaced the rivets with rubber rawl-plugs, but these also failed and then (eventually) I resorted to plastic welding in new fittings, which have held to date.
A lovely design let down by inadequate finishing. Manufacturers of sit-on kayaks really need to properly test their products in the intended environments.