It's generally considered one of, if not the hardest, language for an English speaker to learn.  Per the ratings of the Defense Language Institute (generally acknowledged as perhaps the premier language training school in the world), here are the ratings for English speakers:
Category I: Dutch, French, Italian, Portuguese, and Spanish
Category II: German
Category III: Belorussian, Czech, Greek, Hebrew, Persian, Polish, Russian, Serbian/Croatian, Slovak, Tagalog [Filipino], Thai, Turkish, Ukrainian, and Vietnamese
Category IV: Arabic, Chinese, Japanese, and Korean
Category V:  English (for non-native speakers)
Just to give you an idea, I got sent to training for Arabic at the Defense Language Institute.  It ran 63 weeks.  That's 8 hours a day, 5 days a week of heavy-duty, immersion training by mostly PhD-level native speakers in a cost-is-no-object training environment.  And even then we had over half wash out during the course and less than half of those who completed the training got a passing score, which is a 2 on a 5 point scale. With a 5 score meaning having the fluency level of educated native speaker, a zero score meaning someone who could say "I don't speak your language," read signs intended for the mentally handicapped, etc.  Level two (a passing score) is being able to read, write, or speak on limited, non-abstract topics which would be understandable (although challenging) by a native speaker.