Arabic is generally considered to be one of the easier languages to learn. Nevertheless it still requires a lot of effort and a good teacher to learn. A lot of people try Rosetta Stone when they need to learn Arabic. A lot of other people are considering it but are not sure if it is really the best way to learn.
The strength of Rosetta Stone as a tool for learning Arabic is that it is quite good at teaching you vocabulary. You will be shown four pictures and a voice will repeat the word that describes the item in one of the pictures, you then have to pick the correct picture. This can be a very effective way to build up your vocabulary, unfortunately this is about all that you are going to learn using Rosetta Stone.
The biggest problem with Rosetta Stone as a way to learn Arabic is that Arabic uses a different alphabet than than most of the people trying to learn it are used to. Since there is no instruction offered on the alphabet students need to learn it someplace else. Unless you do know the alphabet the program will become essentially useless as most of the instruction after the basics of building your vocabulary is done in writing. The idea is that by immersing you completely into Arabic you will be forced to learn it but you are going to have to learn the alphabet before you start.
The other issue that you will face with Rosetta Stone is that offers limited instruction in grammar. You will move from vocabulary to forming basic sentences but these are not grammatically correct sentences, they are more like the way a baby would talk. The result is that you will learn how to communicate at a very basic level but the training that you will get beyond that will be limited. If you want to get really proficient with grammar you are going to have to take a more advanced course.
It would be fair to say that Rosetta Stone is an effective way to learn Arabic. It will provide you with a basic vocabulary and if you take the time to learn the alphabet you will learn enough to get by if you visit an Arabic country. If you want to become truly fluent you will need more advanced training but since this is not what Rosetta Stone claims to teach you it should not really be an issue.