I considered Rosetta Stone (for Spanish) but ended up going with Fluenz Spanish instead. The main difference is that where RS emphasizes full immersion, the developers of Fluenz think that adults learn better by leveraging their knowledge of English. Children have the ability to learn by immersion right away, but as adults we can't do this effectively. That's the argument anyway, and it made sense to me which is why I went with Fluenz. I've gone through the first 14 lessons so far and I'm pretty happy with it, although both programs are expensive so this isn't a budget solution either way (aside from torrents).
If you go to the
Fluenz website, they have a free demo available for Mandarin, Spanish, French, and Italian. You get to go through one of the actual lessons online, except that the demo doesn't include microphone support. I believe it's exactly the same otherwise, so you'll have a good idea what to expect.