I've tried Rosetta Stone for a few different languages (well okay, just 2, Japanese and French).
How well do they work? They work really well.
How long does it take for them to work? Right away.
You'll immediately start learning key words and phrases with Rosetta Stone.
I'm not saying it's a magic pill, and obviously you aren't going to be speaking with someone in that language a week or more later. Obviously it takes time to learn a language, I don't care what anyone says, no matter HOW you learn, it will take time.
However, Rosetta Stone and other software programs are in my experience the best way to learn - simply because they are:
1 on 1
Interactive
More engaging than a book / dvd / cd / tutor / teacher (unless you have an amazing teacher, good luck with that for a language class).
Use them when you want.
Two other programs I strongly recommend are Rocket Languages and Fluenz. If I had to rank all three products in terms of overall rating it would be:
Rocket Languages
Rosetta Stone
Fluenz
Fluenz:
Read the reviews: http://www.amazon.com/s/ref=nb_ss_hpc?url=search-alias%3Daps&field-keywords=fluenz&x=0&y=0/?tag=clogui-20
Rosetta Stone:
Read the reviews: http://www.amazon.com/s/ref=nb_ss_gw?url=search-alias%3Daps&field-keywords=rosetta+stone&x=0&y=0/?tag=clogui-20
Rocket Languages:
http://abclearnlanguage.com