A word of warning - arrived at Doha International Airport today and saw no signs or desk for purchasing a visa on arrival. Seems that everybody including myself had a visa prior to our arrival. I was flying in from Beijing and even got asked there if I had a visa for Qatar. Also, even with a visa I was asked how much I had on me.
Just wanted to give you a heads up. I would strongly advise getting a visa before you get to Doha.
Btw, I'm a British national and very relieved to have had a visa before arriving.