The only significant upside of TI's official SDK for TI-68k calculators, TIFS, is the ability to generate FlashApps for TI-68k calculators. But on the TI-68k series, FlashApps have never been as popular as on the TI-Z80 series, because ASM programs had no significant size restriction.
Its downside are many, for instance (but not limited to)
* C compiler limited to a severely outdated (before C89 !) dialect of C and generating bad code;
* slow and buggy IDE which requires Microsoft proprietary Java (incompatible with Vista and 7);
* slow, highly inaccurate and incomplete emulator;
* headers, library significantly less complete than those of TIGCC (which was released a while before TIFS), and therefore GCC4TI (superset of TIGCC).
The documentation is a mixed bag: it's horrible on low-level aspects, but some high-level functions remain undocumented even in GCC4TI (though it has dozens of functions and variables that TIGCC doesn't have).