How difficult is Persian?

Quick Summary:

No vocabulary overlap with European languages.
The Arabic script (unless you want to skip that).
As simple a grammar as can be.

The difficulty of a new language for you as a learner is partly informed by the languages you already speak. In particular, cognates will jump-start your vocabulary. For example, as much as 60% of French words are similar to English. As a speaker of English, you will not find many such cognates in Persian. This means you will have to learn more vocabulary. And then of course there is the Arabic script. It's not as hard to learn as you might think, but it doesn't make Persian easier for you (unless you want to skip it).

But that is where the bad news ends. The pronuncation is very regular - Persian is spoken the way it is written (very much in contrast to the worst offender in this regard, the English language). The Persian grammar is extremely simple and regular. There are no genders (no le, la, der, das). There are essentially no irregular verbs - once you know how to conjugate one, you can conjugate them all. There aren't any grammatical cases either, and the same plural suffix can be used with every word. That is a whole world of sweat and tears you can leave to those unfortunates trying to learn German.

While history has not recorded what Mark Twain thought about Persian, we do have a couple of choices quotes from other notable persons.

By comparison, Persian is absolute child’s play. Were it not for that damned Arabic alphabet in which every half dozen letters looks like every other half dozen and the vowels are not written, I would undertake to learn the entire grammar within 48 hours.

Friedrich Engels
Letter to Karl Marx

Persian Grammar is simplicity itself.

Alexander Finn
Persian for Travellers, 1884

Farsi is essentially Esperanto in disguise. [...] No cases, no grammatical gender, straightforward verb declension - probably my favourite non-european language! Realistically all you need to know is one present tense and one past tense, so drop the class already.