I’ve tried Rosetta Stone a few times, and I find it to be pretty difficult to work with.
It moves at a slower pace than I would like subject wise, but the software itself is very slow. You spend a lot more time waiting on it to load and say the next word than you do actually learning.
My second gripe with Rosetta is that it is not broken up into manageable chunks. It has units, but they last an arbitrary amount of time. Should you do 30 minutes a day? An hour a day? Two hours a day? I never know when I should quit, or if I should quit. Since the lessons build within the unit, the last section is 4 times as long as the first one.
I can say that I did actually retain knowledge gained with Rosetta Stone, but I think my time would have been better spent with another program.