Is it a true 100Hz panel or is the 100Hz just achieved with the TVs processor?
If it is done by processing and bsically adding in virtual frames, it won't look nearly as good as a proper 100/120Hz monitor.
However, if it is true 100Hz, then in fast moving games it will look great 0 though the pixels may look a bit big if you are sitting close to it.
As for TV vs monitor - there is the pixel size problem I mentioned. Also, there are sometime problems with high input-lag, as the TV's image processing system delays the signal getting to the panel. However, if a game mode is available then this can be reduced.