Only the Camel Knows...
I've been studying Arabic for three years now, and I have to admit, I love it. Spoken classical Arabic has a musical poetic quality that is not matched in any other language I know. It has been an intensely rewarding experience, but full of challenges.
The biggest challenge, in classical Arabic at least, is the complexity of the grammar. The rules on grammar are really difficult, I feel. We would often ask my professor about a difficult or obscure rule, or why such a rule even existed, and he would reply "only the camel knows why". However, Arabic has a root system, where any word can be broken down into three, sometimes four, base letters. Meanings can be expanded upon or derived by finding the root and then by looking at the way to root is manipulated. This was incredibly frustrating when I first began my study! Once you get the hang of finding the roots and recognizing the patterns, though, the relationships between words are fascinating.
Another difficulty is the vast difference between classical and spoken Arabic, and then the often vast differences between regional spoken Arabic. Spoken Arabic though is much simpler in grammar and structure. I studied Egyptian colloquial, which is widely understood because of the popularity of Egyptian films and music (which, by the way, is beautiful music, though there is a lot of fun newer music as well). Though it is very different from the classical Arabic I learned in the classroom, it is engaging, distinctive, and incredibly rewarding.
I always have mixed feelings when talking to potential Arabic students. On one hand, it is incredibly frustrating in the classroom. On the other, its a beautiful world language, with incredible history and literature. It is my favorite foreign language to speak and write in, and the best academic choice I think I've made.