Having read contributions on this thread I used myvietnamvisa.com for a recent visit and had no problems after applying online one week earlier, just took the approval letter to the desk at immigration and paid the money. No line at immigration and I was out in roughly 15 minutes though I did arrive at 7.30pm on a half full Vietnam air flight from Singapore mid week.
The airline were cautious that the approval letter missed my middle name but let me go anyway and wasn't questioned in Ho Chi Minh. Probably wise to take care on this in the future.
Not sure if others mentioned it but the visa on arrival fee can be paid in most currencies, so no longer necessary to take USD. For me this is a benefit as it cuts down exchange rate costs and is one less currency to carry around if you are a frequent traveller in asia.
