Life is short. No one wants to fill out a form. Be conversational. Be funny. Gradually engage. Do the unexpected. It is the role of the designer to express their company’s brand to elicit an emotional reaction. If done correctly, it will increase completion rates. Just make sure you don’t violate the rules listed above.
Use the Crystal Ball technique to help you pick the right ways to measure success. Ask yourself “If I could know anything in the world about how people are using my product, what would I want to know to in order to tell me whether or not my product was successful?” (Typically the answer people come up with is not #of clicks on a button but something more abstract like How many people who used my product received value from it?) From that answer, work backwards to get to a measurable metric that best approximates what you’re trying to get at.