Yes. My wife, baby and Myself arrived back on 8th June after 2 of those visas, once the immigration seen them I was grilled as to if I used an agent, both mine an my wife's ignorance to the question sparked the red button. On arrival the boss took the passport and said "it was OK this time because I had a proper Non O, but I was lucky if I hadn't I would have probably been sent home."
So yes get a visa before coming, still doesn't gaurantee entry but greatly increase the odds.