This site provides regularly-updated stats. It pulls them by crawling the League system, not by users submitting logs, so it should be a reasonable random sample.
Check this out:
Champs like Soraka push the win rate up to about 55%, which is impressive. But I think the other end is more telling.
For the past month:
For the past 2 weeks (Sejuani patch):
The custom date range doesn't seem to work, which is a shame.
I have a feeling that some names have been in the low 40% range for months.