Learning salsa fast!?

There's a real danger in overtraining (not to mention bankruptcy) if you get in too big a hurry. Private lessons will have the biggest payoff, especially if you get them from more than one good instructor. Quality of instruction, as noted in the other answer, is quite variable. Some who specialize in salsa alone have no training at all in teaching dance. It's more reasonable to take a few group lessons to start, then add the private lessons, practice (ideally with a partner who's a little better than you), and DANCE. It's important to dance with different people and not get stuck in a rut with people from one group. Oddly, there are bunches who dance well with each other but can't dance with those outside their group!?