I can't say enough good things about Casa Hispana! I took classes here for a 6-month period, and was well conversant at the end of it -- able to do research in Spanish and navigate myself and my husband around Peru.
The way it works is that you sign up for a 10-week course, choose your level and the days/times you want to attend, and they put you into a group of up to 10 people, mostly young professionals. You meet twice a week for 2 hrs each time (except the Saturday classes). The teachers are all native speakers with expertise in teaching Spanish as a second language, and they follow Casa Hispana's own special curriculum. This method focuses on getting students to speak very quickly, making you feel like you're at a chit-chat session and not in class. It is really fun -- not kidding!
There is some homework involved, but probably no more than 20-30 minutes each time.
I think that what makes Casa Hispana work so well (fast learning, great fun) is that the owner, Sonia, has managed to establish a sort of a great family environment. From the type of teachers she hires (all young and fun-loving people like herself) to the coffee and cookies she offers, to remembering every student's name, she makes everyone feel like this is their place -- which provide extra incentive to study hard.
All this comes with a price tag of $395 (I think) for the 10-week course, but it's SO worth it!!!