In their recently published landmark study, Professors Josh Lerner and William Kerr were able to demonstrate that on the whole, angel-backed companies enjoy performance gains of 30-50% when compared to other non-funded startups. Their paper is meticulously researched, invoking in one instance Hammurabi's Code itself to underscore the ancient provenance of individuals making equity investments in high-risk ventures. A great deal of the data analyzed was derived from the actual practices and outcomes of both the Tech Coast Angels (Southern California) and the CommonAngels (Boston).
Their choice of success metrics is interesting. I think I would have chosen profitability or liquidity instead of survival, venture capital raised and web site traffic.