A major difference between HMO and PPO plan is that HMO requires a primary physician's referral before one goes to see a specialist.
In practice, how big of a drawback is this? It does take one extra trip to the doctor office. However, it seems wise to see a doctor who are better than I am at knowing what's necessary anyway. In addition, urgent care is covered no differently between HMO and PPO, so this "drawback" of HMO is not a factor during emergencies when time is valuable.