Zoomerang and Survey Monkey are similar - basically 'off the shelf' tools if you are in a position to develop, execute and analyze your own survey. Interesting that you use feedback both to track on-going trends and for issue-specific feedback on an event. Combined, that information would be very powerful.
To your question, the research on research suggests that, provided you achieve a reasonable response rate, you are probably getting a balanced view (positive, negative and neutral). Some firms worry that they will only hear about the positive or the negative but our experience suggests otherwise. Again, that is only if you have a reasonable response rate and unfortunately what is 'reasonable' changes depending on the size of the population.
When it comes to satisfaction, you often know about clients who are very satisfied and those who may be dissatisfied. They have probably told you already. I'm very interesting in using feedback to identify clients who say they are 'neutral' about the relationship. They would not have told you they are dissatisfied but they are definitely at risk. If you can uncover them early, the client feedback is like an early warning system in the business.
It's also interesting to remember that a side-benefit of client feedback is that you not only get input on satisfaction, but can also identify additional revenue opportunities among clients. There is some interesting research that demonstrates that conducting client feedback can directly impact the bottom line.