Just because a famous VC backs a company, doesn't mean that the company is set for success. Join a company because you believe in their product and their mission.
Most of all VCs invest in companies that end up going under, they are making just as much of a risk as you are in joining the company.
In terms of questions to ask: - What are your company goals and missions? - Where do you see the company in 2 years / 5 years / 10 years? - What was your motivation to start the company? (you want to understand their passions and individual goals)
At the end of the day it's your decision. Go with what you believe in.