When users land on a Search Engine Results Page (SERP), what percentage click on one of the organic results, and what percentage click on one of the sponsored ads?

It really depends on the search query. For obviously purchase-oriented queries, Google displays more ads in an effort to get clicks on ads instead of on organic listings. For example, the paid/organic clickthrough split on a query like "buy plane ticket to new york" could be 50/50 or even favor the paid listings.

However, for a keyword like "fondue recipe," where the user has expressed less of an obvious interest in purchasing something from an advertiser, the split is likely to heavily favor the organic listings.