Working during my Mexican adventures (save one week when J came down) has been wonderful for the obvious reasons: I have been able to spend a significant chunk of time in another country without having to quit a job, live off of savings and risk long-term unemployment upon my return. I'm working 4 days a week, so I have three days every week to enjoy the city (although the addition of classes means I don't have any fully free days).
But those pros come along with some serious cons. This has not been a true immersion experience because I have to spend 8 hours a day, 4 days a week, thinking, writing and speaking in English. That's a must. Obviously, if I were not working, I'd still be riding the English-language wave by checking in with family and friends via phone, email, etc. But working is a real cock-blocker when it comes to improving my Spanish as much and as quickly as I'd like.
That said, I definitely have sensed improvements. I've been able to clear up confusing rules. I've learned a lot of colloquial Mexican-Spanish. And perhaps most importantly, I'm putting myself out there and making mistakes without beating myself up too much. That all has led to more confidence.
A big help in maximizing my Spanish-only time has been music. It's a tool I use in SF, too. Whenever I want to kickstart my day or need to refocus after a long spell of English, I open up my iTunes "Spanish" playlist and force my mind to begin thinking in Spanish. Here are some of my longtime favorites (all of which helped teach me a grammar trick or wicked Spanish-language expression):
Julieta Venagas - Me Voy (Mexico)
El Canto del Loco & Amaia Montero - Puede Ser (Spain)
Leonor Watling & Miguel Bosé - Este Mundo Va (Spain)
Diego Torres & Julieta Venegas - Sueños (Argentina)
La Oreja de Van Gogh - 20 de Enero (Spain)