Iw as there in 2013 and while it looks like they've simplified the visa arrangements I'd still advise you get a visa in advance.
In 2013 the visa on arrival didn't exist and you had to go through a convoluted process of getting 'invited'. If you check the details of the visa on arrival, you still need an 'invitation' ..
So I'd recommend you get this sorted before you arrive, just to avoid disappointment. I'm heading there April 2016 so will update once I find out more.
Having said that, Iran is a fantastic destination and well worth the effort to get to. Highly, highly recommended - go before too much changes. almost 5 years ago