No computer program can change the fact that, for whatever reason, learning a language is hard for adults. Rosetta Stone seeks to emulate a child's learning, but it's possible that the brain has changed to make this much harder as an adult. In The Language Instinct, Harvard psycholinguist Steven Pinker guesses this is because learning language was evolutionarily necessary only in childhood. The brain is a metabolic hog that uses a disproportionate share of the calories we consume—it doesn't make sense to keep around equipment, like the language-learning circuitry, that we won't need later on.
Rosetta Stone will help you build vocabulary and confidence, but it's best used alongside traditional tools like the dreaded Je m'appelle Pierre. Children have a supple language instinct. Adults need to rely on their advantage in cognitive horsepower.