I took the "Music Video Production" & the "Beginning Screenwriting" courses in 2008 at the USC School of Cinematic Arts Summer Program and had an amazing experience! Not only are the facilities top notch ( I was just in L.A. and saw the new facilities they built- WOW!) but the faculty were incredible and I learned so much. Not only did i work on my own class projects but i was able to collaborate on other projects going on in the program and learn even more! I can't recommend the SCA Summer Program enough! if you didn't make it there this summer try for 2010!
The same faculty that teach during the school year teach several of the USC summer courses. The school is yours: there are very few cinema classes during the summer, so you don't have to deal with the degree program students or fight them for equipment. They provide a lot more than they do for degree programs, too, i.e. tapestock.
I can't speak to NYU, but I imagine it's of similar quality, and I have mostly heard great things about the extension programs at UCLA as well. (please don't ask me for specifics, though! )
I know people that have gotten into programs at USC, UCLA, and AFI after taking summer/extension classes. I don't know anyone who's out there getting paid or whatever you measure success by, but that doesn't mean they don't exist...I'm not around campus in summer to meet them.
I have no idea how much they cost, though. If you have the money and you think you can do better for the cost, than go for it.