What PIC24 C compilers are out there, and what is your opinion or review on them?
Microchip's C30 compiler comes with extensive libraries for all the peripherals, and lots of other stuff can be downloaded. I bought the full version but the free version is adequate for most users. All the Microchip code examples use it.

Most people use C30 for the PIC24 and dsPIC. The other compilers don't have a very good reputation, judging from comments on the Microchip forum.