Just started with Fluenz Spanish and think it's working well for me so far. I took a chance and bought all 5 levels as a package and haven't regretted the investment yet. I tried out the demo for Rosetta Stone too, but Fluenz seemed to take a more holistic approach and teach things that will be more useful in real life. I think some sort of immersion course would be awesome, but I'm not able to do that right now. I can't say how long it'll take to get basic conversation skills down as it probably depends on the pace you go through the material at, if you supplement with other sources, and so on. I've read some posts where people have gone through several levels of the material over the course of a few months in preparation for a trip or something.
If you're interested I don't think it'd be a waste of time. I'm sure that depends on what you're hoping to get out of it though. Good luck.