Having analyzed Trinet plans against readily available group plans, I don't think PEOs are necessarily better in terms of health insurance, though that probably depends on how good your broker is.
But I remember having a conversation with a friend-of-a-friend who worked as a health insurance advisor or broker (don't remember which) told me quietly that up to around 250 people, you can generally get much better rates from a PEO than from the provider directly (assuming you negotiate hard), and that she thought more companies should be using PEOs. She also felt that the reasons more companies didn't was having to basically turn over the whole benefits and payroll function to a third-party, and the loss of control therein.
So - again, I'm sure there's variability, and you should look at all your options, but I think PEOs are something that startups should be considering in the mix.
Startups innovating in the space like ours, Maxwell Health (http://www.maxwellhealth.com), are able to offer way better tech and valuable services for startups and do all of it for free. We've been able to save money for companies who were currently using your typical PEO.