I have been toying with French on and off (mostly off) for years. I took a couple of years of it in high school and a year at university but that was so long ago it's barely a memory. Presently, I read it quite often (books and magazines) and also use grammar books for the exercises on various things. I find I can read and comprehend fairly well with the help of an ever present dictionary. However the one thing that has always been lacking is the speaking aspect of it. This is where Rocket French fills the void.
You can hear how the words are actually pronounced rather than try to guess based on the way they're spelled. You will notice as you progress that there are many words that are written the same in French as in English. Some have the same meaning and some do not (faux amis). But in both cases they are rarely pronounced the same. For some reason the French have this strange French accent!!
So this is what I think is the biggest benefit of RF. I can't speak for anyone else, but in learning French I found my mouth just does not work in French. The combination of different sounds, the use of the mouth and tongue (and what's the deal with that rr !!) are all different that the American use of the mouth. RF helps to train the mouth by constant repetition in the exercises. Repeat again and again especially as the sentences get longer.
Now to really learn it, I also suggest what Torusan says above, you need to utilize other sources in addition to RF. Find grammar books that are at your level. Expand your vocab. RF presents a lot of various scenarios ( at the airport, hotel, taxi...) but what happens if you're strolling along the Seine ask someone, "Ou est le musee d'Orsay?" and they don't answer you exactly the way the lesson does? This is why you need to use additional resources. Also, I have found a lot of errors in the RF lessons in spelling and translations. So you need to start to see how the French structure their sentences as well as the words they use to convey the message. ex. "La maison dans laquelle j'habite." This is translated as "The house I'm living in". However the literal translation is "The house in which I live" which is also a correct English translation and one everybody would understand, albeit a little outdated.
RF has many good qualities although it isn't the complete package. I don't think there is any one program that is. As I said above, find other resources to supplement RF but the main thing is to keep on keepin on.
P.S. Torusan, what grammar books would you recommend for the intermediate level I currently am using "Ultimate French Review and Practice."