Slate has a vague stab today at trying to explain why someone might want to spend £100 or so on Rosetta Stone. I’ve tried Rosetta Stone’s German course and wasn’t desperately impressed: you essentially end up with a brain full of vocabulary and no grammar or useful phrases, if you’re lucky; if you’re unlucky, you’ll have a different kind of learning style altogether and none of the images actually sink in, leaving you considerably out of pocket and no wiser.
I personally prefer the ‘Instant’ books, which get you up to a rudimentary level in a short time. Certainly, they help more with reading foreign languages than Rosetta Stone does, unless you’re learning a language with a non-Roman alphabet such as Greek or Japanese – the ‘Instant’ books use transliterations instead of the native script.
There’s a useful amateur podcast on learning languages as an adult that’s far more use than the Slater article, though. It’s called Trying to Learn Spanish, and while later episodes are far more concerned with Spanish language learning, the first few podcasts look at more general approaches to language learning as an adult.