Prototyping is essential for fuzzy front-end product development. The prototyping process answers questions about critical assumptions and supports design decisions, but it is often unstructured and context-dependent. Previously, we showed how to guide novice designers in early development stages with prototyping milestones. Here, we studied the prototyping success perceived by novice design teams. This was done in two steps: (1) teams were asked to assign each prototype to a milestone, a specific purpose, a fidelity level, and a human-centered design lens, and then evaluate the success using a predefined set of criteria. (2) Teams were interviewed about the success of the prototyping process, this time using self-chosen criteria. Results related to (1) show that teams perceived prototyping activities with respect to desirability and problem validation significantly less successful than prototyping activities towards feasibility and solution validation. Results related to (2) show that teams mostly chose success criteria related to how well prototypes supported communication, decision making, learning, and tangibility. This insight may be used to give priorities to further improvement of methods and guidance in these areas.