Visited the Sanjay Gandhi National Park on Dec 31, 2009. Had to wait one hour for tickets to the so called "safari". There were only two buses for the safari and hordes of people.
The safari was pathetic (two lions and four tigers in enclosures) and is way inferior to Banerghatta park in Bangalore for example.
The train ride and boating were okay. No decent restaurant in the park pretincts. The slides and swings for children in the playpark are old and creaky. In general the park is badly maintained.
Go only if you have nothing else to do in Mumbai.