[edit] Also, try asking "how do you like being considered a contractor vs full time with health benefits" -- I've found that perfectly "happy" drivers will give you an earful about this.
The ones that have chosen to participate and be Uber drivers seem to be pretty happy about it (or at least the ones I've talked to).
A lot of the Uber drivers I've talked to are part time. For example, the other day, I talked to a grandmother who literally just liked driving around. Uber X gave her a way to pay off her car, doing what she loved. Whenever she got bored, she would just log off and head home. She had nothing bad to say about her job at all.