It depends. The HMO at the hospital I work at doesn't require referrals for specialists, and the prescription plan and co-pays are cheaper. However, since everything with HMOs is co-pay, there's no maximum out-of-pocket...major medical needs (catastrophic accidents, cancer, etc...) can still be prohibitively expensive, even with coverage.
PPOs, OTOH, tend to have annual deductibles, co-insurance, and maximum out-of-pocket. PPOs are traditionally more flexible with coverage (no referrals needed), have larger networks of providers, and cover expenses out-of-state as well. With my HMO, out-of-state treatment requires it be an emergency and treated in the ER, or admitted. No seeing the local urgent care or PCP for the flu when visiting relatives across the country.
Our HMO covers vision and dental, PPO does not. PPO has a better prescription registry (covers more types of drugs, with better allowable amounts/doses).
In my experience, HMOs tend to have lower premiums compared to PPOs.
So yeah, it depends on your needs.
With my current and last employer, the HMOs made more sense for general health care. The costs are generally better, and MUCH better coverage for dental. The PPO would have been the better option if I anticipated getting hit by a car.
Then there's the whole High Deductible PPO + HSA option...I've never gone that route.