I think it depends on what your standards and goals are. We have tried a bunch: K-12 (then called -- I think -- PowerGlide), Middlebury/PowerSpeak, and Rosetta Stone (not online, but CD-Rom).
Across these programs, we tried Latin, German, and French; I speak the latter two (not fluently, but I speak and read at the advanced college level). I would say that K-12 and Middlebury were passable if what you have is a kid who wants some introductory flavor of a language AND who likes an immersion approach. The Rosetta Stone was IMO useless; it taught a few stock phrases ("The girl drinks the water") without any grammar at all. It soon got very boring.
That said, we have abandoned all of these, because I dislike the immersion approach, and my kids REALLY hate it. They are logical learners; they want verb conjugations and full-on presentations of the cases and so on. The immersion approach is maddening for us, because it presents bits and pieces; at any given time, you can say "I like" and "I liked" but not "he likes" or "he liked" (because the rules aren't explained). At one point, you can say (this is an example from Middlebury French) "The king who plays the flute danced" but not more basic things like, "I would like a cheese sandwich."
We switched to a textbook approach, and my kids took off. The textbook approach does require a parent who can teach at least the basics of pronunciation and grammar.
For clarity: we did K-12 languages at the elementary level; Middlebury/PowerSpeak at the middle-school level; and textbooks at the middle- and high-school levels.
