GPS based VMG can still be "useful" but you need to know its limitations and pitfalls.
It becomes more useful the longer you use it. Keeping a sailing diary with all the usual things and adding GPS tracks will help with any training.
There's more than just polars that can be obtained from GPS data.
It gets even harder when you get to measuring instantaneous wind angle because of wind shear, mast rotation (if you have a mast mounted wind sensor), pitching, etc...
A previous thread ...and another... 
