During the summer, most students at NYU and Columbia go home, so both schools use their summer programs to gain some extra revenue and to fill classes which would be otherwise be empty.
I_ve taken summer classes at both NYU and Columbia and they were not bad at all. In fact, campus will be nice and empty which is a nice change from the hectic fall and spring semesters.
In summary, as long as you_ll be studying something you are interested in and the credits transfer to another program which would give you some kind of terminal degree or certificate, I don_t see any problem with taking part in one of these summer programs.
Have fun!