S2U provides implementation of UI-related S60 APIs on UIQ. That means that:
1) application has to be rebuilt on UIQ from its C++ sources
2) non-UI incompatibilities require manual intervention to the C++ code
3) while standard S60 controls will behave as standard UIQ controls, the hard-coded screen coordinates, etc. will not be "translated" automatically, and thus will require manual corrections.
Nevertheless, S2U makes the porting much easier and faster, and - after the porting is done - you will have to maintain only 1 source code for both S60 and UIQ platforms.
n-Gage games? Why not? S2U package even includes a game as an example. It is also a good example how to cope with my note #3 (about screen coords, etc.)