Support for a few extra features that are disabled by default on GF cards, occasionaly very slightly superior rendering quality... but this is only under very specific instances.
Significantly better performance in CAD/3D Modelling/Mechanical engineering applications etc etc.
The disadvantage, is that some of the features that the Quadro enables support for can reduce performance in a small handful of games, and there may be compatibility issues with an equally small number of games that mis-detect the card as a GF and improperly utilize it's features.
Basically, if your doing any 3D design work then the mod is most definitely worth it.... if your not, then there is no real benefit at all and a few slight disadvantages.
If you want me to be more specific about the features enabled by the Quadro and the relative impact of those I can, but the above is the basic gist of it.