Great questions - the success rates vary tremendously - from less than 10% on many platforms to as high as 42% on other platforms. You can track statistics (neither comprehensive nor 100% accurate, but close) at http://thecrowdfundingcentre.com/
But, what does this actually measure? If you lump together and compare amateurs with zero pre-campaign preparation with agency driven campaigns, you lose any value in the numbers. The field is noisy and with the introduction of major agencies, brands and Fortune 100 companies using crowdfunding platforms, it is going to be more and more difficult for startups to compete.
However, between 60 and 70% of people who "fail" at crowdfunding report that they intend to do another campaign. There are significant benefits to crowdfunding - and the financial rewards are typically the 5th or 6th most important to the entrepreneur at the close of the campaign.
Data from Wharton reports that nearly 90% of projects that met a $5,000 funding goal in the past five years are still engaged in the activity for which they raised funds - this includes community crowdfunding, not-for-profits etc. Early evidence suggests that successful rewards crowdfunding is associated with more viable firms, but more research is needed.